Unable to import mp3s to Music library.

I've just bought a new 2021 MacBook Pro 16". I've transferred my existing music library from an external drive to the Mac's hard drive and all of the music that was in there is working fine.

However I'm unable to import anything new. Whether I try to drag and drop or use file > import in the app, I get the same message: "an unknown error occurred (-54)".

Have also tried dragging the music files to the "automatically add to Music folder" but nothing happens.

The music folder location is /Macintosh HD/Users/(my name)/Music/Music/ITunes/Music and the "keep music media folder organised" and "Copy files to Music media folder when adding to library" boxes are checked.

Would be grateful for any help – thanks!