Make sure you hear 'ready to scan' from the chime after pressing the set button for 3 seconds.
Make sure you hear 'ready to scan your QR code' from the doorbell.
Make sure the chime has a flashing red light when you scan your QR code.
Click the QR code in the app to put it on full screen. Put your doorbell close to your phone and point it to the QR code. Slowly move your doorbell away from the phone to try to scan the QR code from different distances.