Since Android 5.1 has limited system partition space, choosing a smaller package is usually safer:
With the right GApps package, your ancient Lollipop device can still access the Play Store, sync your contacts, and run essential Google apps. It won't be fast—but it will work. gapps android 51
GApps refers to the proprietary suite of Google-branded applications that are not included in open-source Android projects like (now LineageOS) due to licensing restrictions. For Android 5.1, these packages provide the necessary libraries and services—specifically Google Play Services —that allow you to sync your contacts, use location services, and download apps from the Play Store. Popular GApps Packages for Android 5.1 Since Android 5
If you are building a custom ROM from source, you can bake GApps directly into the system image. Download Blobs For Android 5
For Elias, this meant a frantic search for fixes. Was it a permissions error? Did he need to Clear Data