From lock screen - phone displays Couldn't process fingerprint. Please try again. in response to most fingerprint attempts.
For what it is worth the 'Couldn't process fingerprint. Please try again' only happens on my Nokia 8 when the fingerprint sensor is touched very briefly.
It will normally display 'Not recognized' and then 'Too many attempts. Try again later' after ~5 attempts with the wrong finger.
Also please note, the phone should vibrate when the fingerprint sensor is activated. The vibration does not occur when the sensor is touched briefly and 'Couldn't process fingerprint. Please try again' is displayed on my phone.
Did you add a screen protector, bumper, or a wallet that may interfere?
You already tried everything except a factory reset and set up the phone without restoring a backup - which is the last troubleshooting step before sending the phone for repair or replacement.
Mechanical defects happen. I hope you get it sorted out.