it's because you have no index.html page published
your main page to your site is index.html
when you sign up for the subdomain or purchase a domain you do get the placeholder index.html page there and then all you have to do is publish yours over it
Don't ever delete the index.html file unless you make your page index.htm then you delete the index.html file from there and it redirects to your index.htm page
but your main page to your site MUST be index.html or index.htm
so but the 403 error page generally means NO index.html page that it can find