"This webpage is not available"

Are you referring to a multisite? Who is your hosting provider and what kind of server are you using?

yes, i′m, Hostgator

Then contact Hostgator.

trying to do that right now, but it′s a hosting issue?

Well, it may or may not be a hosting issue. For some type of servers and hosting providers, you have to go to the control panel and create the subdomain name then let it propagate in which the subdomain will appear in 24 hours or so.