There is a possibilty that the fingerprint sensor may have become defective i recommend taking this to your nearest UBREAKIFIX or sending your phone through 1 (800) SAMSUNG for a diagnostic or repair f the device is in warranty
I see you're having an issue with the fingerprint scanner. I can definitely try to help you with that! Let's start from the beginning. First, what I'd recommend is to remove all of your fingerprints on the phone so we can start from fresh.
You can follow my step by step instructions if you'd like:
Settings > Biometrics and Security > Fingerprints > Fingerprint 1, 2, 3 etc. > Remove
Now you shouldn't have any fingerprints saved on the phone.
Next, I'd recommend you go to a safe area (just in case you drop your phone) and then take off your phone case so that there's nothing in between you and the fingerprint sensor.
Now hold on to the phone the way you normally do and try to scan your fingerprint until the gray finger print turns all the way blue.*
I recommend adding the same finger again at different different angels and any other finger's you may use.
You may end up with about 3 or more fingerprints and that's ok. It adds to the accuracy of the fingerprint scanner. Your fingerprint should now be working as intended when you unlock your phone.
*If the fingerprint print is not turning blue at all, then your fingerprint sensor may be defective. In that case I'd recommend contacting 1-800-SAMSUNG (1-800-726-7864) or visiting your local Samsung authorized repair location found on https://www.samsung.com/us/support/
I hope this information was useful and helped you with your fingerprint scanner issues.