rnbguy

Thanks for all the helpful comments. I removed all imported unusable vector brushes.
Unfortunately, Affinity does not support Linux. So I am using Windows. The path for Windows is %APPDATA%\Affinity\Photo\1.0\user.

Thanks for all the helpful comments. I removed all imported unusable vector brushes.
Unfortunately, Affinity does not support Linux. So I am using Windows. The path for Windows is %APPDATA%\Affinity\Photo\1.0\user.

How about Preferences, Miscellaneous, Reset Brushes? (Or pressing Ctrl while starting and choosing Reset Brushes.)
If those work (as I suspect they would) that would be simpler than mucking about in the file system.
(Of course, it will also reset the pixel brushes, which perhaps you were trying to avoid.)

The message itself is accurate -- the brushes are in fact imported successfully -- but they are inaccessible because there is no vector tool in Affinity Photo that can use them, so Affinity Photo never should have allowed the import in the first place. That's the bug.
I only mention this because the imported brushes are added to an existing Affinity Photo support file & therefore use file space you might want to reclaim for something more useful. I don't know of any way to do this from within Affinity Photo itself, but there is a way to do this on Macs, & probably an equivalent way on Windows.
The basic procedure is to quit Affinity Photo (important!), then find & delete the support file. On the next launch of Affinity Photo, it will automatically create a new file that amounts to a minimum sized placeholder.
On Macs, the name of the file is vector_brushes.propcol. (I assume the propcol extension stands for property collection or something like that.) It is one of the several files in the ~/Library/Containers/com.seriflabs.affinityphoto/Data/Library/Application Support/user/ folder, where the ~/ prefix is the standard UNIX abbreviation for the user home folder. Note that if you have Affinity Designer installed, there is file of the same name at a similar location, so make sure you navigate to the Photo one & don't delete the Designer one by mistake, because that is where all your Designer vector brushes are stored!
A simple way to get to this folder on Macs is to use the "Go to Folder" item on Finder's "Go" menu, & enter the folder path name just as it appears above, or even simpler to copy it from this post & paste it into the text field in Finder. (Finder understands the ~/ prefix.) Click the "Go" button, locate the vector_brushes.propcol file in the Finder window, & drag it to the trash. You can then launch Affinity Photo & it will create the minimum sized placeholder file. Empty the Trash to finish reclaiming the space & your are done.
The procedure for Windows should be the same, but obviously the folder location won't have the same path name.