Sage 50 Prepay "Invoice # already exists"

I am trying to enter a prepayment for a customer using the prepayment check box, as we have received the money but the product has not shipped.

I click the "Apply to Revenues" tab and enter info.

I get an error saying "The Invoice/credit memo number already exists for this Customer", but there is no invoice in the system for this customer.

Please help. I cannot finish bank rec without this payment being entered, as the money has already been deposited into our bank.

Thank you.