How do I turn off screen overlay in Samsung Galaxy S6 Edge plus?

I bought a Samsung Galaxy S6 edge plus in Seoul and took it to Ghana. The phone can make and receive call but almost all apps do not have permission to access the phone's storage. When I try to give permission, I get the following error message:
"Screen Overlay detected. To change this permission setting , you first have to turn off the screen overlay from settings > Apps."
But when I get to settings I don't see how I can turn off the screen overlay. Due to this problem...

How do I turn off screen overlay in Samsung Galaxy S6 Edge plus?

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