Most of the Screen off/lock applications do not support fingerprint unlock once locked. Unlike them, this app will truly lock the screen similar to power button action.
All you have to do is open the app and configure to enable accessible services for the app to trigger the lock screen action. Devices running Android Pie (9) or above supports this true lock action.
Traditional screen off/lock uses the device administrator setting which disables the fingerprint unlock option. This app will not give that disappointment for you.
If the app still showing that it is not configured even though accessibility service is ON, reboot the device once configured.