Hi Loopy,
Welcome to Affinity Forums :)
To work with the inpainting tool you must have a pixel layer selected in the layers panel (make sure you don't have a mask or adjustment or any other layer type selected there). Look at the label between parenthesis after the layer's name in the Layers panel to identify the type of layer you are working with. If case you have dropped (from Finder on a Mac or a Windows Explorer window if you are using Windows) or placed an image to the canvas it may be labeled as an Image layer type - if that's the case right-click the image layer in the Layer panel and select Rasterise. You can then use the Inpainting Brush Tool with that layer.
Image layers are considered object layers - you can transform them (scale, rotate, apply adjustments etc) but they can't be changed at a pixel level. They retain the original image data so you can transform them without losing quality. If you need to change them at a pixel level (for use with the Inpainting or other pixel based tools) they must be converted to pixel layers as explained above. Pixel layers are where you can perform any type of pixel based/level editing.
Regarding the "tools" on the top of the screen, these are actually documents (document tabs) that are currently opened in the program that happen to be named with the tool or functionality that's being explained in the video - so they are NOT tools at all. The names of these tools appear there because the documents were saved with those names to facilitate locating the correct document while the video was being recorded.
