First, for clarity, your urls.py file does not generate a page. It’s only the mechanism that your server process uses to find the view which does generate a page. In this case, the view is the sitemap utility.
But, most importantly, what you’re seeing as a response to the sitemap.xml url is not an error.
The intent of a sitemap.xml file is not to produce a human-readable page - it’s designed for use as an api endpoint for automated retrieval and processing. The fact that you’re getting that message just means that the browser doesn’t have any predefined stylesheet for that file.