Why do I keep getting the error message App was not installed: apk problem?

A

AC Question

App was not installed: apk problem

Ok so... I made a theme for this app called SAO Launcher and all I did was take an already made them and simply change some pictures in it. Didn't mess with the code other than changing the app name and package name. After that, I tried making an apk and after a whole 2 hours (first time making an apk and using Android Studio) I managed to make the apk. I had trouble at first with the key store and after with the gradle. In the end though, I managed to make it and it's called "app-release.apk". Now, I tried moving it to my phone through dropbox but I keep getting the "app was not installed" error and before you ask, yes I do have "unknown sources" checked so that's not the problem. Please help!