QuickBooks Online Sync Error: Reference/Check number for this payment is already being used in QBO. The payment may already be recorded in QBO.
Why the Sync Error occurs
This error occurs when a payment has a reference number (Ref#) that is already in use in QuickBooks or a payment for multiple invoices has a Ref#.
How to Fix the Sync Error
There are two options to resolve this issue:
If the issue is due to a single invoice, edit the payment to update the Ref# to a unique number
If the issue is due to multiple invoices, edit the payment to remove the Ref#