GT70

Thank you very much! With your procedure I solved it. I can now switch from Designer to both Photo and Publisher.
No problem on Publisher, as well as on Photo to switch to Designer.
I would never have got there on my own!

And anyway, thank you all for your advice!

Can you try a basic reset of Designer, to do this shut down Affinity Designer and then start it from Applications while holding down the Control key and you should get shown a screen with a number of tick boxes on and 3 options will be ticked and you can just click on Clear.  Designer will then start as normal, are you now able to use the Edit In Photo option?
Do you also have Affinity Publisher installed? StudioLink is different from the Edit In command, but it would be worth seeing if StudioLink reports any problems when trying to switch to the Designer persona or if that reports a problem

@walt.farrell
Thank you for your advice. 
Yes, Photo and Designer are both at version 2.3.1.
Both are in the applications folder and open regularly without any problems.
I have found that, on the other hand, from Photo to Designer the switch option is available...
So I don't know why this behaviour occurs. 
@GarryP
Thank you for this trick! 
I thought I could edit the photo from the "Persona Designer" view and since I couldn't, I thought I had to use Photo.
I have to decide to study the manual....
Thanks again!

@walt.farrell
Thank you for your advice. 
Yes, Photo and Designer are both at version 2.3.1.
Both are in the applications folder and open regularly without any problems.
I have found that, on the other hand, from Photo to Designer the switch option is available...
So I don't know why this behaviour occurs. 
@GarryP
Thank you for this trick! 
I thought I could edit the photo from the "Persona Designer" view and since I couldn't, I thought I had to use Photo.
I have to decide to study the manual....
Thanks again!

Hi @GT70,
When converting a document from a regular canvas which had column or regular guides to an artboard, there's a known bug where even after deleting the guides on the artboard that 'ghost' guides still remain that are visible when marquee selecting. These guides aren't visible in the guides manager as they are drawn on the canvas which is no longer in focus and the only way to get rid of them is to revert the document back to a canvas by deleting the artboards, which isn't exactly feasible on multi-artboard documents. What's shown in your recording appears to be the same or a similar trigger for this bug, so I'll bump the existing issue with your report.
Unfortunately I'm not aware of another workaround to remove these 'Ghost' canvas guides in an existing document, as deleting the artboard they originate from will just re-position them again. If you create a new empty artboard document, you could try copying and pasting your existing artboard layers over to the new document and hopefully the 'ghost' canvas guides won't follow as they do not exist on the hidden canvas.

@thomaso
Thank you for your pointers, eventually I will do as you say by scaling everything proportionally so that it all matches. At least I hope so.
@debraspicher
Thank you for your very professional explanations. My English is not as good as it sounds and I think I only understood part of your explanation. 
In any case, I attach the screenshot of the total window of the Sparkle working environment. Obviously having a monitor set to a higher resolution, if you measure the whole image, you will not get the 1920px you are looking for. The part you are interested in is the part indicated in width by the yellow box that measures 1920px.
Above you can see that Sparkle is set to 100% and that the working area of the page is 1920px (which is the space where the yellow box is)
I hope this solves the mystery...
PS: sorry, but for privacy I had to blur parts...
 

Don't feel bad as it is often overlooked.
Your screenshots just made it clear to me to point it out and to resolve your problem.

You have that rectangle nested in another object, but I'm not sure what kind. Possibly another rectangle? 
Is that really what you intended, and if so what were you trying to accomplish?

Select the slider for opacity and set it to 100%

I believe there is an issue with the indentation, which for some reason has recorded a non-pure black. In fact, if I position cursor before the start of the sentence, the color changes and then becomes black on the first useful character.
Who knows what happened. Lesson learned though... next time, I will set the text with the correct color from the beginning.

Hi @GT70,
It should be sufficient to select the text box without highlighting the text itself...
My best 'guess' is that when you've been highlighting some of the text you may have missed the very first character in your selection. In that instance, you can have a situation where highlighted text appears as K100 but the text box itself appears as a CMYK mix, though your screenshot doesn't seem to indicate that.
If your text uses text styles then you should be able to edit the text styles to apply them universally across your document.
Happy to take a look at your finished document or your exported pdf to check everything is correct if you'd like us to...

@lacerto thank you very much for help! Knowing that there are people willing to help you is something extraordinary.Yes, I will now make the changes and before sending the file I will post a selection of pages here on the group for checking. It will take me 3/4 days, but as soon as I'm done I'll post here and ask for opinions.

@Hangman I will do as you say and then, as written to @lacerto, I will post a selection of pages from the finalised file here. Thank you!
PS: Yes it is Baskerville.

Hi @GT70,
Amazon KDP is a bit of a mystery, they provide little useful information, certainly no information about colour profiles but I think that is likely because they strip out any colour profiles anyway.
From a bit of research I did a while back I believe Amazon use HP Indigo digital presses to print their material... If you want to upload a couple of pages from the corrected pdf that you've resubmitted to Amazon we can take a look and check all is as it should be.
The Assign and Convert options in the Document Setup window are not relevant in this particular instance as you're not assigning or converting anything. Your Publisher document is set using CMYK and U.S. Web Coated (SWOP) V2 so as long as you maintain those settings, then set all your black text using the CMYK sliders in the colour palette to K100, i.e., CMYK 0, 0, 0, 100 and export your file using the PDF/X-1a:2003 preset your file 'should' be fine but again one of us can certainly check it for you if that helps.
The font in the sample pdf you uploaded is a bit odd, it's showing as

rather than Baskerville which is I believe the font used...

That is perfectly understandable, and a route that I would probably prefer to take myself in any situation. It is always better to get a proper grasp of things oneself than feeling dependent. The production issues, however, can get absurdly complex within Affinity apps and long experience (both generally within the industry, and in using Affinity apps) can help to spot the actual issues in a snap, so save time. So if you get desperate, feel free to change your mind and contact, or just continue exchanging your experiences on more abstract level here on the forum. There are plenty of capable users willing to help you.

I think there is always a hint of sadism in the Affinity Designer guys...I lost a whole day figuring out how to export. Every time they make an update I'm afraid I'll never find what I know...maybe it would have been better to write "with background" and "without background"...
Thank you very much, you saved my day. I'd buy you a coffee if I could! Consider it done!
Thanks again!

Thank you walt.farrell, I solve with transparent background.
 

At the cost of basic functions that are missing? No, my friend, that's not going far. I repeat, if you don't listen to the users, you're not going anywhere...
This post was opened on the 21st of February, 2018. In about 5 weeks it will be 2 years since the request for a tool for distortion, perspective and warp. Two years my friend are GEOLOGICAL ERE in the computer field. 2 years, not 1 month! Shameful.
The great thing is that no one from Affinty puts his face here to tell us what's what. As far as I'm concerned, if this is the treatment, I don't buy their products anymore.

Find new solutions out there, all right. But I think that "running away" to other products is a defeat for everyone, starting from Affinity that continues, despite our complaints, to do nothing.
For my part the result is not having bought Affinity Publisher and not having bought Affinity Photo even with the discount.

It is certainly not a reward that can save a company from disappointment of its users. I don't think I'm asking for the Moon or are we all wrong? That Affinity is rewarded is fine, but I repeat, it must be interested in the requests of its users because if they exist it is thanks to those like us who have paid for their software. The day you add the features we need, we will all be very happy. At the moment we are not.
Don't worry, I'm currently locked into Adobe which allows me to do things that Designer doesn't do at the moment.
Then as you've already seen from other people's interventions here, new alternatives start to appear on the horizon and Affinity was once an alternative too...
You'll see how pulling too much rope doesn't get you anywhere.
Or maybe it just brings a prize...
 

It is certainly not a reward that can save a company from disappointment of its users. I don't think I'm asking for the Moon or are we all wrong? That Affinity is rewarded is fine, but I repeat, it must be interested in the requests of its users because if they exist it is thanks to those like us who have paid for their software. The day you add the features we need, we will all be very happy. At the moment we are not.
Don't worry, I'm currently locked into Adobe which allows me to do things that Designer doesn't do at the moment.
Then as you've already seen from other people's interventions here, new alternatives start to appear on the horizon and Affinity was once an alternative too...
You'll see how pulling too much rope doesn't get you anywhere.
Or maybe it just brings a prize...
 

We didn't tell him to make 3 different software, to go on Mac and Windows and also on iPad. The desire to expand has to be faced with its own strengths. If they have 5 programmers, it's useless to jump on several fronts without completing important functions. First you complete a product and then move on to another product. Where is the sense in creating so many incomplete products? We are here to support them and we could buy all the products they make but there is a parameter called "SATISFACTION" and "CUSTOMER ATTENTION". If you' re asked to create a function, Affinity team have to stop eating and do everything  to create that function (the other one is basic). Instead they do not consider you at all and so the answer is that people like me will not continue to buy their products because they are afraid to buy other incomplete products...
 

At the cost of basic functions that are missing? No, my friend, that's not going far. I repeat, if you don't listen to the users, you're not going anywhere...
This post was opened on the 21st of February, 2018. In about 5 weeks it will be 2 years since the request for a tool for distortion, perspective and warp. Two years my friend are GEOLOGICAL ERE in the computer field. 2 years, not 1 month! Shameful.
The great thing is that no one from Affinty puts his face here to tell us what's what. As far as I'm concerned, if this is the treatment, I don't buy their products anymore.

I'm sure Affinity is prepping big updates in 1.8 - with these features and others we've been begging for. We know they've been improving the expand functionality for example... They have us all frustrated with the slow pace of necessary additions. I've been hanging on to CS6 and using that most of the time for the things I need that Affinity lacks, but I always worry Adobe will find some way to deny me my license rights if the computer crashes and I need to reinstall it. Knowing there are options like vectorstyler helps me feel better if the worst case scenario occurs.

(...)
 

You're absolutely right. This software promises a lot but then has its legs cut off because there are several things missing to make it really a great softawre to the point that many of us have to go back to using Adobe Illustrator if we need to work with the free transform tool.
I think it's really absurd not to listen to what users have been asking for for a long time as they support you and encourage you to improve more and more.
Honestly, I'm not interested in iPad versions (which I bought) or versions for publication (like InDesign).
I have the impression that they waste of effort and creates a lot of software that is not 100% complete. I think they should first complete one and then switch to the other.
So I add myself to the appeal: when will you insert this blessed function?
Or is the alternative back to Illustrator?
Sorry for my bad english