Biometrics: When I try to register my fingerprint in Settings, the screen is too dark. See photo.
Brightness hi
Brightness high, auto brightness off.
So can't do it! Nothing happens. What next? #A71
Do you have a screen protector fitted at all that maybe hindering the sensors @Members_3YVOgKv
If not then you may have to have it looked at via a Samsung Service Centre or from where you acquired the phone from.
Sometimes a factory reset after backing up can help in some cases.
Support links >
https://www.samsung.com/uk/support/mobile-devices/my-fingerprint-scanner-isnt-working/
https://eu.community.samsung.com/t5/galaxy-a-series/a71-fingerprint-problem.
Fingerprint Scanner Troubleshooting.
I wish you all the best. 
