Why the sync error occurs
This error occurs when a payment cannot sync to Sage Intacct from Bill.com because Sage Intacct doesn't allow the sync to use a bank account that's 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 Sage Intacct, under Cash Management > Credit Card Accounts. View each Charge Card account and be sure the Credit-card offset account isn't set to the payment account used on payments in Bill.com.
The best practice here's to create a separate bank account in Sage Intacct to associate those charges to.