Why the Sync Error occurs
This error occurs when a bill with billable line items is updated in Bill.com after the bill has synced to QuickBooks Online (QBO) and the invoice for the billable item(s) has been created in QBO.
QuickBooks Online does not allow changes to bills with billable item invoices linked to it.
How to Fix the Sync Error
Manually update the bill and invoice in QBO and contact Customer Support to remove the error.