Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
Go into itunes while phone is plugged in.
You will see the tabs such as:
Summary, Info, Ringtones, Music.
Select the 'Music' and where it shows
un-select or un-check that option
and click or tap on
in the bottom right, and then once the sync is complete re check the music sync option.
This is said to work to free up the error you have seen. It means temporarily not syncing music, but the next sync including music may work. This error may possibly come of trying to sync both music and other content.
Let me know how it goes, please.