When you try to install Lets Encrypt service on your domain you get an error ,
urn:acme:error:unauthorized :: The client lacks sufficient authorization ::
Invalid response from http://yourdomain.com.
This plugin gives you the ability to return this specified url to Let's encrypt service for the right functionality of the service in nopcommerce store.
Just install the plugin and nothing else.
After the installation you can put your Free SSL to your domain.