Your Sitemap appears to be an HTML page. Please use a supported sitemap format

I submitted https://lagatar.com/sitemap_index.xml to Google WMT. It gave 1 error (Your Sitemap appears to be an HTML page. Please use a supported sitemap format)

Then i submitted:-
https://lagatar.com/post-sitemap.xml – no error
https://lagatar.com/page-sitemap.xml – 1 error ( Your Sitemap appears to be an HTML page. Please use a supported sitemap format )

What should i do?

https://www.ads-software.com/plugins/wordpress-seo/