Why the sync error occurs
This error occurs when an updated or voided payment is no longer linked between QuickBooks Online and Bill.com.
How to fix the sync error
Most of the time, a payment void is trying to sync to QuickBooks Online. In those cases, simply void the payment manually in QuickBooks Online as we can't resync a void from Bill.com.
If it's a payment update attemtping to sync, it's still the best option to update manually in QuickBooks Online, but you can opt to delete the payment in QuickBooks Online instead, and contact Customer Support to resync the entire payment to QuickBooks Online from Bill.com if you choose.