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