As painful as it sounds, Google will continue to break APK downloads on 7.1.2. Many custom ROM teams still offer stable Android 8.1 Go Edition for legacy devices. You will gain better Play Store compatibility and security patches.
The Play Store relies on Google Play Services . If your Services version is from 2018, the Store will not function. Download the corresponding Play Services APK for Android 7.1. Google Play Store Apk Download Fix For Android 7.1.2