MP3tag shows the covers that are embedded in the files.
The Windows Explorer uses a hidden file called folder.jpg
You can import picture files with an action.
Select the files for which you want to import covers.
Open the function Actions(Quick) and select an action of the type "Import cover from file".
format string : *.jpg
(this imports the first picture file that is found in the folder)
Format string: folder.jpg
(this imports the file folder.jpg if it is present).
To check which files still have no picture use a filter (Press F3 to see the filter input box at the bottom of the window)
Filter: %_covers% MISSING
For me it is not at all clear where you have your folder.jpg-files. Normally someone wants a cover of an album to show. So are your folders album-oriented? Or are all your music-files together in directories like "2000's"?
From the error-message you get I can only see that you obviously wrote "Format string:folder.jpg" as a format string instead of just "folder.jpg".
You can enable MP3Tag to also show not only embedded cover-art but also cover-art that only resides in the folder if you leave
"Don't display first image ..."
in File -> Options -> Tags
(Please enclose text with backslashes and other special characters in accent grave (top left to bottom right) - this shows the text as it is and does not interpret parts as control characters)
The main thing is: the filename and that what you enter as filename REALLY have to match.
It is virtually impossible for me to see from the distance where the differences could be.
The basic approach is correct: enter a fully qualified filename if the cover files are stored in a single folder. I see the pattern of
as a source for trouble.
Is there really a file called Cover ArtMadonna - Like a Virgin.jpg?