If you're getting a "Screen overlay detected" warning when running WazzapMigrator the first time, it means you have some other app running with "Draw over other apps" permission enabled.

screen overlay detected

When an overlay is running, Android will not allow any new permission to be granted (such the ones requested by WazzapMigrator) thus resulting in the “Screen Overlay Detected” error.

How to fix: this How-to Geek explains how to get down to this for each Android manufacturer (like Samsung, LG, etc).