error when trying to view sitemap

Hi,
I installed yoast seo on a site and activated sitemaps. When I clicked to view the site map I got the following error:

Not Found

The requested URL /sitemap_index.xml was not found on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

Apache/2.4.12 (Unix) OpenSSL/0.9.8e-fips-rhel5 mod_jk/1.2.37 mod_bwlimited/1.4 Server at http://www.rockdaledems.org Port 80