Want website using server host name domain?

So I have named my DA server, server.domain.com and now I want to create a website on that domain.

When I try to add a user or reseller with that domain name I get "Already Exists" error.

How do I add this domain so I can have a website on it.

Try adding the domain name to your admin account.
Login as admin, then on the right side of the page click on "user level" and add the domain there. Then you can host a website on it.

As far as I know you can't give the domain used for the hostname to a user or reseller, only to admin.
If you want to create a domain for a user or reseller, you have to use another domain name. Or like stated above, use the admin account to create a website for that domain.

Make sure that you don't have domain.com in DNS or under another user's account.