You’re talking about setting up a redirect domain, correct? And is it a 404 page from your site or a generic error?
Let’s say I have a site at example.com, and I want to add example.net as a redirect; visiting it will immediately change to the .com version.
1) Ensure that example.net is set to point to the same server as example.com via DNS settings; make sure the IP address is the same.
2) Ensure your server is setup to accept traffic from that domain. With a simple redirect domain you technically don’t even need this plugin, just configure it in cPanel/Plesk instead.
At this point, see if the domain redirects to your network’s main site. It should be routing to the wordpress install, and WordPress is redirecting to the official domain the site uses.
3) In WordPress go to My Sites > Network Admin > Domains and click Add New
4) Fill out the form and hit Save Changes. This is mostly needed if example.com isn’t the default site on your network.