Non appstore app won't install

[Android Central Question]

I'm trying to download an app (one that I'm currently making at the moment so I know for a fact there's no viruses) from my files but if I click install it starts and then a second later it says app not installed? It works on my friends phone but not on mine.

 

[B. Diddy]

Welcome to Android Central! Which phone? "Unknown Sources" is turned on, right?