On your Android 5.1.1 device, open Chrome (or any browser) and go to: https://www.apkmirror.com/apk/google-inc/google-play-store/
This usually means the APK you downloaded requires a newer version of Android (like 6.0 or higher). Double-check that the APK is compatible with Android 5.1. Google Play Store APK Download for Android 5
Running an older device with Android 5.1.1 (Lollipop) can sometimes lead to issues with the Google Play Store, such as the app not being pre-installed or requiring a manual update to restore functionality. Manually installing the is a reliable way to get your device back into the app ecosystem. Where to Find Verified APKs Play Store version: 18