A customer doesn't have a card
Before you receive a payment from a customer using the Receive Payments window, you must create a card for the person or company and enter at least one order or invoice for the customer in the Sales window. (Adding cards)
If this is a one-time customer and you don't wish to create a card before receiving the payment, you can use the Receive Money window in the Banking command centre to make the payment instead. (For all your regular customers, though, we recommend that you use the Receive Payments window to ensure that payments are matched to the invoices they're paying, and to provide more accurate reporting on your business relationships.)