I\'ve read every StackOverflow that I could find on this issue, but I still can\'t work it out.
I\'m trying to deploy an app to actual device (NOT emulator)
Finally managed to find a solution to this illusive bug.
It seems that if I didn't have the Windows Phone 8.1 Tools installed I couldn't build to an ARM device (not even Windows 10 Mobile).
So my step to solve this was: Go to Control Panel -> Programs and Features -> Microsoft Visual Studio ... 2015 -> Modify. And then select the Windows Phone 8.1 Tools.
Pretty pissed that one can't get a reasonable error message when things like this is missing.