ipod classic

See Corrupt iPod Classic - Apple Community. What are your HDD stats? If you're running macOS 10.15 Catalina then you may find you need to restore your device before you can sync it through Finder. In particular Catalina doesn't seem to work correctly with Windows formatted iPods.

tt2

iTunes still works with iPod classic. So too does the new Music app in Catalina. What exactly is the issue you'd like to resolve?

tt2