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:
Once you have verified your site with Google Webmaster tools, Google Sites will auto-generate a sitemap xml file. Your sitemap will be generated at:
http://sites.google.com/a/(your domain)/(site name)/system/feeds/sitemap for Google Apps. http://sites.google.com/site/(site name)/system/feeds/sitemap for sites under sites.google.com/site
Once these Sitemaps have been created, you can submit them to Google using Webmaster Tools.
So why is a Sitemap file that is auto-generated by Google seen as an HTML by Google?
Googler, JohnMu said that in this case, it appears to be some sort of bug. He said he would alert the Google Sites team about the issue. JohnMu said:
You can ignore that error message -- this is your homepage's URL just written in a slightly different way (without the trailing "/"). I'll pass a note on to the Google Sites team about this, but it's not affecting the processing of your Sitemap file so you don't have to worry about this :-).
Even though Google is telling this webmaster that the Sitemap file is bad, Google is actually eating (indexing) the contents of the file with no problem.
Forum discussion at Google Webmaster Help.