The error occurs because you've got HTML code which shouldn't be there at the beginning of the sitemap.xml, and this makes the XML invalid.
The HTML code is this part:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html><head>
</head><body>
<br>
<br>
</body></html>However, I'm not sure why the HTML code is there.
Do you have a cache plugin activated in your WordPress instance?