I recently made an android app. It worked just fine on both my development devices when installed from Android Studio but when I sent the GDrive links or the APK files to my friends for testing, "App not Installed" error showed up. What's worse, I can't even install the APK directly even on my devices without installing it from android studio. Why is this happening? Please help.
You should build a debug APK. For this follow this
From Android studio above menu select Build -> Build Bundles(s) / APK(s) -> Build APK(s)
Then wait some time. After build success, you will find your APk here
\app\build\outputs\apk\debug\app-debug.apk
Then share this APK and IT will work fine.