Why the Sync Error occurs
This error occurs when updating a bill in Bill.com when the bill came from Intacct and the origin of the bill was a Purchase Order. You are not able to edit bills that originated from Purchase Orders in Bill.com or in Intacct.
How to Fix the Sync Error
To update bills that originated from Purchase Orders, you have to update the actual Purchase Order in the Purchasing Module in Intacct. This will update the bill in Intacct and then a sync will bring the update to Bill.com.