Your navigator calls the root of the Web site or a repertoire (URL ending in one "/", without specification of file name).
The error message is due to the fact that the server does not find a file of index (index.html, index.php). Initially, check that the repertory public_html or the traversed sub-directory has the good permissions (at least 705) in order to make it possible to your navigator to read the page.
If the permissions are correct, check the presence of a file index.html or index.php in the repertory concerned.