How to resolve "package file is invalid" error in google play store?

[AC Question]

I am in big trouble. My device is Phicomm E653. I am not being able to install any apk file whatsoever because of the error "app not installed". I get "package file is invalid" error after downloading an app from play store. All of this persists even after I factory reset my phone. Cleared google play store cache and data. Tapped on play store build number which returned no results as to update playstore. And now I am stuck because I cannot install any apps into my phone and my device is useless.