Why do I get this message: "Error: This domain name has already been taken. Choose another." when trying to add my domain name to my site?

Why do I get this message: "Error: This domain name has already been taken. Choose another." when trying to add my domain name to my site?

Check to see if this domain name has been added to any of your other sites. If the problem still persists, email us at support@zohosites.com.