Screen overlay detected error message
SDK 23 users may get an Android pop up message that a screen overlay was detected and that they should change the permissions. This is an Android 6.0 bug. When Heads Up Notification appear and some alert is visible, Android reports about the screen overlay. Only Google is able to fix the issue.
Some third party apps (e.g. Clean Master) are known to block the overlay function. So it's best to remove such apps.
Specific case Samsung
Solution is to disable the “One- Hand Mode”: Android Settings → Display and Background → One-Hand Mode