The reasons for the error and how to solve it:
/name_site / www /if we are talking about a subdomain - in
/name_site / subdomain /.
.htaccess, check if it already contains a redirect to another site directory. In this case, accordingly, the index file will be searched specifically in this directory.
.htaccessrestriction of access to the site can be set or access to the directory is closed. You can remove this restriction by removing the lines that are responsible for access restrictions from this file, or by commenting them out by putting the symbol at the beginning of the line