If you still need to print the label:
Office stores such as Office Depot or FedEx often offer printing services.
Sometimes your local library may have a printer you can use for a small fee.
If you are able to drop off the package directly at the Post Office:
You can select the “generate a QR code” option on the app.
Scan this QR code at a self-service kiosk or directly with a postal worker, and they will print it 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!