So I've been through "Music" (great name for an application /s) and set the conversion options, then tried to actually convert some audio files - but all that happens is that unhelpful message. Tried it with m4a and m4b audio files, and all the output options that are available.
Try Repair security permissions for iTunes/Music for Mac - Apple Community. Music might not convert read only files otherwise I'm not sure what the issue is. Which particular instance of Music on what macOS are we talking about?
tt2
You cannot convert protected media, or items that are in the cloud.
tt2
Local files and unprotected - using iTunes on a Windows PC I could easily convert them, but no luck with "Music" on a Mac.
"None of the selected files could be converted"
