Android 5.1 uses API Level 22. Modern Facebook apps target API Level 30+. You must find a legacy APK that explicitly supports API Level 22 or lower.
Using an old Facebook APK on Android 5.1 is a compromised experience: apk facebook android 5.1
Follow these precise steps to safely configure your device and install the application. Step 1: Enable Unknown Sources Android 5
APK (Android Package Kit) is the file format Android uses to install apps. Sideloading an APK means installing software manually, bypassing the Google Play Store. bypassing the Google Play Store.