You don't see the bill you want to pay
If a purchase transaction doesn't appear in the list, it may still be a quote or it may have been fully paid already. To see if it's been fully paid, mark the Include Closed Purchases option. All fully paid (closed) purchases will appear in the list.
To see whether the transaction is still a quote, open the Purchases Register window and click the Quotes tab. If it appears there, highlight it and click the Change to Bill button. Then it will appear in the Pay Suppliers window and can be paid.