Domain Mapping - Domain Name Is Unavailable To Access

I've just installed multisite for the first time (actually the second time because I started from scratch with I couldn't get the Domain Mapping to work yesterday).

Basically I get this error whenever I try to map a domain.

My main site is
I have the following site on the network:

I was trying to map (which I own and have pointed at my hosts nameservers) to the site when I get that error. Trying to map any other domain returns the same error.

I know this questions gets asked all the time in the forums, but I've read many posts and can't figure out what's wrong with mine.