Android 5.1 typically runs on devices with 1GB to 2GB of RAM. Messenger can be resource-heavy. Apply these tweaks:
For daily text and voice on old hardware (e.g., Samsung Galaxy S4, HTC One M8, Moto G 2nd gen), use Messenger Lite . Download the final Lite APK for Android 5.1: Messenger Lite v133.0.0.5.75 (available on APKMirror). Messenger Apk Android 5.1
A previous version of Messenger is still installed, or the APK is corrupted. Solution: Android 5
When the Play Store indicates your device is "not compatible," you may need to sideload a specific APK. Always use reputable repositories to avoid security risks: Samsung Galaxy S4