QuickBooks Online sync error: Reference/Check number for this payment is already being used in QuickBooks Online. The payment may already be recorded in QuickBooks Online.
Why the sync error occurs
This error occurs when a payment has a value in the Reference number field that's already in use in QuickBooks Online or a payment for multiple invoices has a Reference number. The Reference number field in Key CashFlow syncs to the Check number field within QuickBooks Online. QuickBooks Online requires that the value in the Check number field be unique for each payment.
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#