A message tells you to use another window for your payment
This message is for your information only, and you can continue entering the payment if you like. To ensure that the correct accounts are updated and your reports are accurate, however, it's best to use the window suggested by the message in most instances. For example, if you select a customer card, the message may suggest that you use the Settle Returns & Credits window if you're paying a credit amount on a previous transaction. The Spend Money window is best suited for payments that aren't related to any earlier transactions. For example, you could use this window to print a cheque for a contest prize, or to pay a one-time supplier for office supplies.
Remember that it's not necessary to include a card on your transaction. Be sure to include Payee information, though; it generally is required to print the cheque properly.