Settings > Apps > All > Google Play Store → Tap → Clear Data → Clear Cache .
On modern Android phones, the Google Play Store updates automatically in the background. However, on Android 5.1.1, this process often fails due to "signing conflicts" or server-side deprecation. Settings > Apps > All > Google Play
Because you are installing an app from outside the Play Store itself, Android security features will block the installation by default. on Android 5.1.1
F-Droid or AuroraOSS’s official site.