When you submit your sitemap in Google Webmasters and it gives the error –
“Your Sitemap appears to be an HTML page. Please use a supported sitemap format instead.”
Sitemaps, at the core of it, is just a collection of links and related description to various parts of your site. It’s just a way to tell the people, and search engine spiders about the location of the content which they’d otherwise miss. But these days, sitemaps are mostly created for the benefit of search engine spiders.
Google may not accept HTML sitemaps; and I really don’t know the reason. Maybe they need a more formal mechanism of telling what the links are, when were they created, what should be the priority with which they should be visited and what’s the frequency that the content on these links will change.
You may submit multiple sitemaps or just a single sitemap with links pointing to any part of your domain or sub domain. Technically; a sub-domain may be an entirely different site; and Google may not pass the page-rank to subdomains from the main domain. That’s why people choose to list their own separate sitemaps for each sub-domain
Google webmaster tools gives an error when you are submitting sitemaps because you can use HTML sitemap,Google webmaster tools can support only XML Sitemaps.So you can create XML sitemap by using following websites.