Why the Sync Error occurs
This error occurs when the transaction being modify in QuickBooks is already in use by another user or the QuickBooks database has temporarily locked the transaction from editing or updates. It's how the system protect multiple users from modifying the same transaction at the same time.
How to Fix the Sync Error
The first step is to see if other users in QuickBooks are editing transactions. If so, request that they complete their editing. Once the edits are saved, try to sync again. If you continue to see the error, switch the company file to single-user mode, close QuickBooks, and sync again.
Once complete, run a manual sync.