If you receive the error 'Your Sitemap appears to be an HTML page' when attempting to submit an XML sitemap to Google Webmasters Tools/Search Console, it's likely that you are using rewritten links for the sitemap and you have some third-party plugins preventing the Joomla! format to be recognized correctly.

As a rule of thumb you should NEVER activate rewritten links for the sitemap unless there is any particular reason such as disallowed raw links, etc. So the first thing to do is ensure that the following parameter is turned off and that sitemap links in the SEO Dashboard are generated in the standard raw format:
https://www.mysite.com/index.php?option=com_jmap&view=sitemap&format=xml


Always submit standard raw links to Google Webmasters Tools/Search Console in order to avoid errors and failures. As an alternative, if you really need to use rewritten links for the sitemap, you can enable the parameter 'Force format for SEF links' so that the Joomla! format will be recognized correctly:


Finally ensure that you have not installed any third-party SEF extension such sh404sef, JoomSEF, etc. In particular if you are using sh404sef configure it as reported at this link: https://storejextensions.org/faq/can-jsitemap-work-with-sh404sef.html