Sub domain problem

So I set up my subdomains to point to my root /htdocs/ folder, but when typing in the subdomain in my mobile browser I get a page that says no index file, Any suggestions?

I usually recommend to disable MJ (by disabling corresponding system plugin) and then archive work of the site by both main domain and subdomain (that is subdomain displays identical page). In most case it's done by set DocumentRoot for subdomain identical to the value for main domain. After that you can enable MJ and use this subdomain for mobile version.