Official support for Android 5.1 Lollipop ended years ago, and critically, Google Play Services support for these APIs was discontinued in mid-2024. This means the standard "Update" button in the Play Store may no longer work for your device.
If you need to install or reinstall Gmail on an Android 5.1 device, you must use an of a legacy version that still supports the "Lollipop" architecture. Android 5 Lollipop | Specs, reviews and EoL info - InvGate gmail download for android 5.1
Downloading Gmail for Android 5.1 (Lollipop) is becoming increasingly difficult as Google and the Play Store have shifted support to newer operating systems. While the latest versions of Gmail now generally require Android 6.0 Marshmallow or higher, users with older devices can still access their email through specific workarounds. Official support for Android 5