Primarily for older phones
I only encountered the
App not installed error when trying to install an apk on my phone which runs on 4.4.2 aka KitKat, but my friend did not encounter this error on his phone which runs on 6+. I tried the other solutions such as removing the old/debug version of the app because the apk was a release version, clearing the debug app’s data, and even clearing all of my cached data. Then, finally I realized all I had to do was select both signature versions when building my signed apk.
Before I only had
V2 (Full APK Signature) selected, but after selecting
V1 Jar Signature as well, I was able to successfully install my signed APK on my 4.4.2 device. Hope this helps others!