QuickBooks Online Sync Error: Bill/Invoice number is being used on another bill from the same vendor. Use a unique invoice number for the vendor bill.
Why the Sync Error occurs
This error occurs when a new bill or invoice is trying to sync from PayMaker to QuickBooks Online (QBO) but the invoice number is already in use for that vendor or customer in QBO.
How to Fix the Sync Error
Edit the invoice number in PayMaker to be unique for that customer or vendor, and sync.