Authenticating and installing biometric data for login on iPhone

I have successfully installed fingerprint recognition in my iPhone several times. It usually works on the first try, but than after a few days, it no longer works. I get the message that I must be authenticated to add or delete a fingerprint. When I search for "authentication on iPhone," I get information about how to find my phone's IMEI, which doesn't help me at all. Apparently the purpose of that is to make sure that I have a "real" iPhone. I do. I am confused - I just want to use fingerprint recognition.