This usually happens when duplicate customer or sub-customer records exist in QuickBooks Online. If BQE CORE is mapped to a duplicate or inactive QuickBooks Online record, payments cannot sync successfully.
To resolve this, review and consolidate duplicate customer or sub-customer records in QuickBooks Online. Make any duplicates with no transactions inactive, and reassign transactions from other duplicates to the correct parent customer or sub-customer. When only one active customer or sub-customer remains, return to CORE, un-map the affected project, and remap it to the correct active QuickBooks Online record. After remapping, the payment should sync successfully without errors.