I can't put music in the ipod shuffle 4th

I installed the latest version of iTunes on Windows 10 pc and synced it with my iPod. No problem up to this point, but when I try to put the music in the iTunes storage box on my iPod, it says "Because I can't copy the Apple music song to the iPod." Can you tell me a solution? I'm Korean, so I'm not good at English, so I used a translator. I don't know if I can get my words across to you guys, but I need your help. Thank you