Android 7.1.2 is old; Play Store may be outdated.
You've tried everything. If the Play Store still won't download APKs on Android 7.1.2, you must face reality: Google Play Store Apk Download Fix For Android 7.1.2
Android 7.1.2 uses WebView to render in-app purchase screens and download dialogs. If WebView is broken, the download button does nothing. Android 7