The XMPP/Jabber Server requires to create an account before you can use it with a client.
XMPP allows to create an account with your client, but unfortunately, this feature gets (ab)used by bots to create thousands of spam-accounts.
Thats why this feature got deactivated on this server.
You need to register an account manually before you can use it. Click on the Domain you want to use with your new XMPP account to get redirected to the registration form: