Why Can't I Install any 3rd party apps on my S7 edge?

[AC Question]

I've bough my S7 edge last year and it was totally fine the whole time. Last week, I tried installing an apk but the install button didn't seen to respond. So I tried rebooting it and the problem happens to be still there. I tried safe mood and I was able to install that time.
I can only install if I'm using the safe mode and not the normal mode.
Is there any solution for it and why isn't the install button not working?
Thanks in advance.
I'm running andoird 7.0 btw

 

[Tim1954]

The phone is protecting itself and you by blocking sideloads. Generally is a really bad idea to do that.
However, if you must, settings... Lock screen and security... Unknown sources... Allow.
The consequences are yours if you end up hacked, you have been warned.