Sitemaps that you submit to Google are usually in XML format:
See http://www.sitemaps.org/ for details.
And often have a filename of the form: "Sitemap.xml"
These are machine readable, not intended for end users. They simply inform Google about all the pages on your site.
However, Google is reporting that the sitemap you have submitted is in HTML format. Sitemaps for humans to read are usually in HTML.