If the fingerprint option is greyed out...
At least for this phone, Samsung Galaxy S6. It seems that you have to have your phone setup so that you use the fingerprint scanner to unlock the phone. Once I enabled that, the fingerprint icon was then lit up. I was then able to setup/configure the fingerprint login in KeePassDroid. After I set it up, I was then able to login to the data using my fingerprint scanner on the phone.
Since I don't use a password, pattern, etc. to unlock my phone, I decided to set the phone unlock method back to "Swipe". Even though the fingerprint info is still in the phone as I use it for other apps, the fingerprint functionality seems to rely on the fingerprint phone unlcok being your method of unlocking your phone. Once I turned off the phones unlocking security method, the fingerprint method would no longer access the Keepass database. I did, however, manage to get it working with the phone set to unlock using the pattern method...just not the swipe method.