When you received this error, it typically means that you haven't added an A record for the domain you're trying to reach, or that it hasn't propagated yet. Check your DNS and you should be all set. For reference, this is what the screen looks like:
At Namecheap, even if you have the DNS right, it will sometimes not propagate until you re-add a root A record as an @ sign instead of the domain name. We're not quite sure why that is happened, but we've noticed it with several Namecheap customers.