Your Sitemap appears to be an HTML page. Please use a supported sitemap format instead.
The issue is, the Sitemap files are auto generated by Google Sites. The help document explains:
Your sitemap_index.xml is in the page cache of W3 Total Cache. I would just exclude the sitemap_index.xml and all other sitemaps (post-sitemap.xml, page-sitemap.xml, category-sitemap.xml, post_tag-sitemap.xml,ngg_tag-sitemap.xml ) from the page cache. Hsbte result
Hope this helps
