I have a sitemap that is supposed to be in my root directory

But when I click on the link... www.mywebsite.com/sitemap.xml it just takes me to the forum with an error notice

Sorry, there is a problem

Cannot find the page you requested

Error code: 2T187/2

 

How do I fix this so I can see my sitemap?

The sitemap file for version 4 is sitemap.php and what you linked is the sitemap for version 3, sitemap.xml which won't work any longer.

You can still view sitemap.php in the browser. it contains links to other sitemaps which search engines know how to follow.

The old sitemap.xml can be removed or renamed if you wish.

Be sure to update the URL in your Google Analytics account (if you use it), to reflect the php vs xml file name.