If you were required a Face ID or passcode, just hold your iPhone near to the reader, making the top of your iPhone lining up to the center of the reader. No need to double-tap with the side button.
If you were required a passcode, and no choice for using Face ID. It may be the result that the Face ID scanner cannot recognize you for times.
If you having a face covering, and the screen is on and you just looking at the screen without unlocking it, a unsuccessful biometric match attempt just made. A passcode or password is required if there were five unsuccessful biometric match attempts. Details at Touch ID, Face ID, passcodes, and passwords - Apple Support.
You could try:
- Reduce the action of switching screen on without unlocking it.
- If your iPhone is unlock while you want to use the Express Transit, no need to lock the phone/switch off the screen, jus hold near the POS directly.
The upcoming iOS 15.4 will bringing new feature to iPhone 12 series and newer models, allow unlocking with a face covering. Let's look forward to it!