'App not Installed' Error on Android

Question

I have a program working in the Android Emulator. Every now and again I have been creating a signed .apk and exporting it to my HTC Desire to test. It has all been fine.

Answer 1

Using Android Studio, I had previously installed the unsigned debug version of the APK (Build > Build APK) and had to uninstall it before installing the signed release version (Build Variants > Build Variant: release, Build > Generate Signed APK).