GrabzIt can only take screenshots of URL's that are publicly accessible like google.com. GrabzIt will not be able to take a screenshot of anything located on localhost. An alternative is to use GrabzIt's IntraProxy.
If you are calling a new domain it may not have propagated to all domain servers yet and so may not be possible to lookup. If this is the case use the IP address of your server instead until the domain has fully propagated.