Hello there, jill16.
The Receive Payment won't generate a transaction number unlike other sales transactions. You can enter your own reference number or numbering system.
If you processed the payment outside QuickBooks, you can copy or use the transaction number from where the payment was created. Then, enter it on that box.
The transaction or reference number will help you easily locate a specific transaction. From the toolbar, click the Magnifying Glass. Enter the reference number to search for it.
Please let me know if there's anything else I can help.