When sitemap not found

I am using yoast plugin and created a sitemap named as sitemap_index.xml. I have run an audit for www.diebestengutscheine.de that shows the error message that "sitemap not found". What do to resolve this problem?
Thanks in advance....

First of all, let us be clear that this is usually not due to our plugin, but due to poorly coded themes and/or other plugins. A 404 error basically means that the sitemap index or individual sitemap does not exist for a variety of reasons.

A 404 error basically means that the sitemap index or individual sitemap does not exist for a variety of reasons. You can solve the 404 error by following these steps: Permalink structure. Missing content.

If your Sitemap.xml is not found, that's something you should immediately address as it’s really important. You want to fix that right away and fortunately, it's very easy to do and I'll show you how to do that right now.