ERROR: sitemap_index is not a valid feed template.

It’s most likely a plugin conflict. I had the same issue with a custom plugin. Try making all the plugins inactive, except for WordPress SEO and activate one by one. Once you know what plugin is causing it, then it’s much easier to diagnose.

See if this post helps:
https://yoast.com/xml-sitemap-in-the-wordpress-seo-plugin/