Cause: Facebook has deprecated the server API version your APK uses.
Fix: This is a hard blocker. You must find a slightly newer APK (if available for 5.1) or switch to Messenger Lite.
This is where the review splits into two realities. Messenger Apk Android 5.1
1. Low-End Devices: If you are running Android 5.1 on a budget phone from 2014 or 2015, Messenger is a heavy lift. The app is notorious for its RAM usage. On a device with 1GB or 512MB of RAM, Messenger will likely cause system lag, slow down multitasking, and drain the battery significantly faster than standard SMS apps. Crashes are not uncommon when switching between the camera and the chat thread. Cause: Facebook has deprecated the server API version
2. High-End Legacy Devices: Surprisingly, if you are running Android 5.1 on a more powerful legacy device (like an old Google Nexus 5 or Samsung Galaxy S5), the app actually performs quite snappily. The Lollipop aesthetic (Material Design 1.0) actually blends somewhat well with Messenger’s UI. Messages send instantly, and the core chat experience is stable. This is where the review splits into two realities
To run Messenger on Android 5.1, you must install an older, compatible version of the app using an APK file (sideloading). You need a version from late 2020 or early 2021.
Recommended Version: Messenger 290.0.0.15.104 (or any version between 280 and 295)
Using a compatible APK for Android 5.1 means you are missing out on the last 3-4 years of feature bloat.