After I started syncing my Apple Music to my iPhone via Finder app there was an error that I was getting over and over again.
The error was ‘Some items, including “Song’s Title”, were not copied to the iPhone “John Appleseed’s iPhone 11 Pro” because they could not be found.’
Error is self-explanatory i.e. referenced files are missing from library and you have to relocate the files by clicking on ‘!’ exclamation mark before song title. Or another option is to right-click on album or song and choose ‘Delete from Library’.
It will just solve the problem.
Hope it helps,
Thanks & Regards