You just installed the very popular YoastSEO plugin. When clicking the on the sitemap XML link you discover a 404 error on the page that takes you to www.yourdomain.com/sitemap_index.xml
Before you dig too deeply into the issue and start to modify .htaccess or any code. Try to disable the XML plugin and then re-enable. Most of the time it will fix the problem on a new install.
