Need help syncing an iPod with a Mac Air M1

Hi, I just bought an iPod Classic 5th gen.

I'm trying to sync it with my Mac, but after a few seconds, an error appears saying that the song couldn't be copied because the file can't be found or read. I have an M1. I don't have a subscription to Apple Music. I only have a 50Gb folder of Music (.m4a) that I imported into the Music app on the Mac.

What do I have to do? 🙏🙏