@Weilong Welcome to the Netlify community.
You don’t use an HTML file for your sitemap. You can use a sitemap.txt file, which is basically a text file with a listing of all the URLs in your site, or you can use a sitemap.xml file, which is a specially-formatted file with more information.
The TXT version should be easy to generate. The XML version requires special construction.
<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
<url>
<loc>http://www.example.net/?id=who</loc>
<lastmod>2009-09-22</lastmod>
<changefreq>monthly</changefreq>
<priority>0.8</priority>
</url>
<url>
<loc>http://www.example.net/?id=what</loc>
<lastmod>2009-09-22</lastmod>
<changefreq>monthly</changefreq>
<priority>0.5</priority>
</url>
<url>
<loc>http://www.example.net/?id=how</loc>
<lastmod>2009-09-22</lastmod>
<changefreq>monthly</changefreq>
<priority>0.5</priority>
</url>
</urlset>
You can build it yourself or use one of the many free online generators or use the Netlify plugin (assuming you can figure out how to set it up), etc.
Either way, you then include the path to your sitemap in your robots.txt file, and register your sitemap file with Google, Bing, etc.
For more information, see: