The app just shows a QR code when I try to scan. What should I do?

It means that you are not using a scan-capable device to scan. Scanning is only possible with an iPhone with FaceID. This is the case starting with the iPhone 10(X).

If you want someone with a scan-enabled device to do a scan for you, they can use the QR code to transmit the scan to you and you can view it on your device.