Why the sync error occurs
This error occurs when a bill is attempting to sync to Bill.com from QuickBooks Online, but there are one or more bills with the same invoice number for the vendor in Bill.com.
How to fix the sync error
Compare the bills in Bill.com to the bills in QuickBooks Online and make sure they all have unique invoice numbers. Edit the invoice numbers as needed and sync again.
If you don't see any duplicate invoice numbers, please contact Customer Support.