If you don't have a printer, you can often find printing services at office stores such as Office Depot or FedEx. Sometimes your local library may have a printer you can use for a small fee. Any of these options is a great way to print a label.
You can also select the “generate a QR code” option on the app. Show this QR code to the post office, and they can create a label for you. If the QR code doesn’t work, try re-selecting “generate QR code” while at the post office, which should refresh the request!