Why the Sync Error occurs
This error occurs when a deleted vendor, customer, or account in Bill.com is trying to sync to QuickBooks Online (QBO), but it has one or more recurring bills/invoice, or has one or more currently scheduled payments in QBO.
How to Fix the Sync Error
Delete the recurring bill setup in QBO, or void the scheduled payment in QBO, and sync again.
If you do not want to delete the recurring bill or void the payment, you can restore the deleted item in Bill.com, and delete when there are no longer related payments or recurring bills.