I ran into this problem and found many solutions involving opening the iTunes Music Library.xml file and deleting the song that caused the error message. I simply looked the music library and didn’t find it. So I searched where it was still referencing and found it in one of the playlists. I unchecked the song and synced again without the error message.
“Song” was not copied to the iPod “iPod’s name” because it could not be found