Since you are not downloading from the current Play Store, you must allow sideloading.
Instead, I can provide you with a that covers: play store app download old version 10.1.08
: Older Android operating systems (such as Android 4.4 KitKat, 5.0 Lollipop, or 6.0 Marshmallow) may experience lag, crashes, or high memory consumption with the latest Play Store builds. Version 10.1.08 is optimized for these environments. Since you are not downloading from the current