Last Updated on Dec 26, 2022
Unknown Sources is disabled in your security settings.
Either you are trying to install the APK from an EXT partition with insufficient permissions or you are trying to install an APK that is incompatible with your Android version (e.g. installing Facebook Home on Gingerbread).
All the buttons are greyed out? That’s because the app is an administrator. You need to disable that setting for that app in your security settings.