Do you get any other error messages? Its hard to say what could be causing the issue, as "app not installed" is a generic message Android gives when one of a number of different issues occur.
What version of Android are you using? The official Godot export templates support Android 4.0+, so if your android device is running an older version of Android, you may need to compile Godot targeting a lower Android version (which may have other issues, I'm not sure)
Disclaimer: I am not an Android developer and have very little experience with Godot and Android.
