Why the Sync Error occurs
This error occurs when a payment being voided or updated is no longer linked between QuickBooks Online (QBO) and Bill.com.
How to Fix the Sync Error
For the vast majority of these errors, the payment is being voided, and the void is trying to sync to QBO. In those cases, simply void the payment manually in QBO as we cannot resync a void from Bill.com.
If it is a payment update attemtping to sync, it is still the best option to update manually in QBO, but you can opt to delete the payment in QBO instead, and contact Customer Support to resync the entire payment to QBO from Bill.com if you choose.