I am trying to map new domain to my subsite

Hi I am trying to map newdomain.com to mydomain.com/subsite. I have an A record created in the NS and is re-directing newdomain.com successfully to mydomain.com. However when I go to the Domain Mapping settings in mydomain.com/subsite site I can't add the mapping. I get the error Domain name is unavailable to access. Anyone have any suggestions?