For Android 5.1 (API Level 22), you need a version of the Play Store that is compatible with "Lollipop" or older.

: Users appreciate that modern versions of the store (even on older OS versions) save bandwidth by only downloading the necessary parts of an app update rather than the whole package. Common Issues :

. Released in 2014, Android 5.1 introduced essential features like Device Protection

Go to Settings > Security and toggle on Unknown Sources to allow installations from outside the Play Store.