A fun one. I modified my Android app and tested it on real hardware (Nexus 5x) with no issues. Later I went to install it on my Nexus 7 tablet: It started into the install and then errored-out with "App not installed" each time I tried. That error message tells me NOTHING!
Found the answer here:
The key part: The app was uninstalled for MY user but not for ALL users on the tablet. I had to go to Settings->Apps->name-of-app. Shows "Not installed for this user". Click on the appname there, click on the 3 option dots in the upper right, select "Uninstall for all users". After that the app can be installed using any of the normal methods. This problem occurred because I had another user set up on the tablet for the kids gaming on trips. The phone only had the one user.