sitemap.xml

Within the plugin settings, the sitemap location reads

The URL to your sitemap index file is: https://mywebsite.com/sitemap.xml

When I follow this link though, I get the following message:

This page contains the following errors:

error on line 2 at column 6: XML declaration allowed only at the start of the document
Below is a rendering of the page up to the first error.

However, somewhere while looking around I see the sitemap:

https://mywebsite.com/sitemap.html …which seems to load a working sitemap page with numerous sub-sitemap pages which upon clicking on the links direct to individual sitemap pages with my blog posts.

Should I be trying to fix the sitemap.xml and using this URL in webmaster tools, or should I be using the sitemap.html?

If I should be using sitemap.html, I’m just curious why within the settings it links to sitemap.xml?

Thanks in advance,

Dustin