It could be a few things: 1. Make sure to check the 'Unknown Sources' box in security settings. You said you already did this, so it's likely 2, 3, or 4. 2. A corrupt APK file (or damaged in some way) or a version incompatibility, either of which would cause an error message. Make sure the app is designed to run on Marshmallow. Assuming it is, download it again in case it was corrupt/damaged the 1st time you downloaded it. 3. If you have any light filters or screen-dimming apps (Luxlite, Twilight, Bluelight Ray, BLFilter, etc) disable/turn them off temporarily for the duration of your app install. Android disallows installing while a filter is active. Some screen dimming apps work by overlaying the whole screen with a translucent window, and letting touch events pass through to the activity below, but for security reasons, you can't interact with system dialogs (such as a package installer) through another activity. Android should tell you that, but it doesn't. 4. There's some transparent activity running, capturing your touch events. Could be a legit app you installed, could be something nefarious. Closed all open apps, then the install button should become responsive. Hope that helps.