This happens when <file> in index <file> line in /etc/nginx/conf.d/foo.conf is missing in /var/www/html/
Example:
server { listen 80; root /var/www/html; index index.html; server_name example.com www.example.com; ...}ls -l /var/www/html/contact.html info.php status.htmlAs you can see, index.html is missing