Why the Sync Error occurs
This error occurs when a payment cannot sync to Intacct from Bill.com because Intacct does not allow the sync to use a bank account that is linked to a credit/debit card.
How to Fix the Sync Error
Unlink the payment bank account GL (COA) from any credit/debit cards in Intacct, under Cash Management>>Charge Card Accounts. View each Charge Card account and be sure the Credit-card offset account is not set to the payment account used on payments in Bill.com. The best practice here is to create a separate bank account in Intacct to associate those charges to.