The sitemap_index.xml does not load the Yoast sitemap. Instead it appears to load a static file that was likely created by another plugin or manually using a generator.
Please remove any static sitemap files as Yoast SEO does not create a physical file on your server. Your webhost can help you remove static files.
If you do not find any static sitemap files, we would like to rule out any plugin or theme conflicts that may occur. The fastest way to do this is to?deactivate all non-Yoast plugins and switch to a standard theme?like?Twenty Nineteen.?
Test this on your development or staging site, if you have one. If not, we recommend using the?Health Check & Troubleshooting?plugin. This plugin allows you to run a conflict check?without affecting normal visitors to your site.
As we can imagine that you’re unfamiliar with checking for conflicts, we’d like to point you to a step-by-step guide that will walk you through the process:?How to check for plugin conflicts.