If your iPod previously used on a Windows computer isn’t syncing with a Mac, you need to check the Disk Format on your iPod.
On iTunes, go to Devices and click on the iPod. It will display the Name, Capacity, Software Version, Serial Number and Format. The format can be PC or Macintosh and needs to be equal to the computer you’re using.
To change the Disk Format on your iPod, click on the button Restore
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.