None of the previous post solve my issue. Here is what's happening with me:
I normally load the app from android studio by clicking on the "Run" button. When you do this, android would create an app that's good for debug but not for install. If you try to install using:
adb install -r yourapk
you will get a message that says:
Failure [INSTALL_FAILED_TEST_ONLY]
When this happens, you will need to rebuilt the apk by first clean the build, then select Build->Build APK. See the image bellow:
This APK is ready to be installed either through adb install command or any other methods
Hope this helps
David