Sitemaps XLM Error

Hello,

I am having some problems with the sitemaps:

I have activated the Sitemaps XLM functionality on the Yoast SEO settings, but once I click on “You can find your XLM Sitemap here: XLM Sitemap” this message pops up with a red background:

This page contains the following errors:

error on line 3 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.

What am I supposed to do? I have tried to add the sitemap to the Google Search Console but it wasn’t able to work.

Thanks in advance,

Ale

PS. My website is: http://frisifont.altervista.org/