installation - Why can't I press the Install button when installing applications from unknown sources?

I'm trying to sideload an application, as seen in the picture below.

I have allowed the installation of applications from unknown applied actions in settings, but I can't seem to press the install button. The cancel button works correctly and installing applications via adb install works correctly. What's the problem?

EDIT: For those who have this problem, using ChainFire's C.F. Lumen will correct it because the app uses hardware compositing, and Android doesn't notice a "layer" on top of the current screen.

Answer

Looking at your screenshot, I think you are using screen filtering apps like Twilight, f.lux, CF.lumen and bluelight filter. Try to force stop/uninstall/disable the screen filtering app and it will work again.

If you are not using any filtering app, try looking for your installed apps that have overlay capability, apps that can show their contents over another apps, apps with overlay permission (like- Messenger, Musixmatch, Touch Assistant apps that can float on your screen).

If you are not sure, try disable them one by one in your Settings ⇒ Apps and find which one is causing the problem. You can also look for "Draw over other apps" menu in your android setting where all the apps with this permission will be listed. If you couldn't locate the menu, search for overlay permission + {android version} + {device manufacturer}.