The reason why "The app you're trying to install isn't a Microsoft-verified app" window is appearing when you try to install Easy MTD is because Windows is in S mode.
Windows S mode only allows applications from the Microsoft Store to be installed. Easy MTD is not one of those applications. However, you can turn S mode off so you can install applications outside of the Microsoft Store. Click here for instructions about how to do this.
When the above actions have been performed you can install Easy MTD in the normal way.