I have a Spotify playlist, and there are occasionally songs that aren't on Spotify, such as EPs, old music, and stuff like that. That's not my problem though (obviously). I can add/switch directories and select files to edit the tags/metadata of, but it won't let me write it to the file. There's a screenshot attached to this showing it. I've restarted my computer and run the software as admin, and it worked once, but when I re-opened the software to add the cover, it stopped writing to the file.
Did you test if it's the Spotify Player that locks your files?
Damn, you're right! I closed spotify and turned off local files and then it worked perfectly fine. You're amazing dude