Created this subdomain on my hostgator server
travel-usa.airportbyappointment.com plus placed in in the behavior tab for home page, saved and published, but cannot access home with that and receievede this 404 not found URL /TRAVEL-USA/sapphire/main.php was not found on this server
address only access is through folder option airportbyappointment.com/travel-usa
In order for SilverStripe to run correctly with subdomains, youll need to make sure that all the domains point to the same wwwroot - that is, both airportbyappointment AND travel-usa.airportbyappointment.com should have their wwwroot set to where you installed SilverStripe.
Well, you can just leave your existing installation in place. Just make sure that all domains you want to run off it point to this location for the document root. Otherwise youll get a 404 like that when it tries to load sapphire/main.php from a different directory.