Why the Sync Error occurs
This error occurs when a bill or invoice is trying to sync, but an account on the bill or invoice has been deleted, or the connection has been broken. The deletion may have occurred in Bill.com or QuickBooks Online.
How to Fix the Sync Error
Check the accounts on the bill or invoice to verify that the accounts are active in both Bill.com and QuickBooks Online. If needed, restore the deleted accounts, or select different accounts.
If the accounts are active, or if the error continues after you restore or replace the accounts, contact Customer Support.