Why can't I press the Install button when installing applications from unknown sources?
Looking at your screenshot, I think you are using screen filtering apps like Twilight, f.lux, CF.lumen or bluelight filter. Try to force stop/uninstall or disable the screen filtering apps 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 &quot;Draw over other apps&quot; 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}.