downloading kingroot
It's been a year, so this isn't necessarily for op, but for anyone with the problem. I just had this problem and fixed it. 

Even with unknown sources ticked, I was still getting a warning message. I just pressed okay without looking, and the app wouldn't install.

Instead, when you get that warning just press more info, then press install anyway. It's smaller text and a different color--easy to miss.

That should do it.