A supplier doesn't have a card
Before you make a payment to a supplier using the Pay Suppliers window, you must create a card for the person or company and enter at least one order or bill for the supplier in the Purchases window. (Adding cards)
If this is a one-time supplier and you don't wish to create a card before making the payment, you can use the Spend Money window in the Banking command centre to make the payment instead. (For all your regular suppliers, though, we recommend that you use the Pay Suppliers window to ensure that payments are matched to the bills they're paying, and to provide more accurate reporting on your business relationships.)