wordpress multiple sites

It looks like you haven’t set up the DNS entries for the sub-domain, so there’s no record of where it’s meant to point to.

You’ll need to do this either with your hosting company or your domain registrar, depending on which of those manages yoru domain names DNS entries. As a guess, most hosting control pansl allow you to set up sub-domains in your account and these will automatically set up the DNS entries that are required, so I’d start looking there.