Display screen overlay is a primary trouble maximum marshmallow and nougat are facing presently. It is a new feature Android added to their latest Os. The pop up message is simply stressful that it renders your mobile phone useless as it denies your apps permissions.

What is display screen overlay?
It makes apps to display info on pinnacle of other apps. For instance, facebook messenger makes use of this feature for their chat heads to be proven over other apps.

Method 1
Download this file from playstore button unlocker open it then tick accessibility then, the screen overlay will not pop up again.

Method 2:
Go to settings>>Apps then click on the little setting gear at the top
Choose "draw over other app" then you will see a list of apps causing screen overlay
Just tap on it one after the other and tick OFF button.

Method 3
This method works for all techno
http://www.techfashy.com/2017/06/how-to-permanently-fix-screen-overlay-on-android-devices.html?m=1

Jus save urself d stress and go to running apps under ur settings and force close or stop some of ur running apps..

Button unlocker app, is probably the easiest way.......