Getting 404 for sitemap_index.xml ?

I am trying to submit my sitemap to Google Webmaster and this is the errot I am receiving.

I read here
https://yoast.com/xml-sitemap-in-the-wordpress-seo-plugin/
that it could be due to W3 Total Cache – which I *do* have installed.

But I have the latest version and cannot see how to add the suggested link as an exception.

Any help greatly appreciated! Thanks!

https://ww.wp.xz.cn/plugins/wordpress-seo/