MULTISITE - MULTI DOMAIN: Activation link 404 not found


I have a multisite and activated the multi domain plugin. In signing up choosing my main site ( activation works fine but when I try to join choosing the second domain ( added in the settings - multi domain the activation link is 404 not found:
Not Found

The requested URL /wp-activate.php was not found on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

I followed carefully the instruction when I installed the multi domain plugin.

Where did I get wrong?

Also wordpress should be also installed here in order to make it work or not? this second domain is totally blank as you can see.

The second domain should be in the same server/cpanel? should I add it here as a addon domain or park it here?

Your help is much appreciated!

Best Wishes,