Sitemap Error ; Xml declaration is allowed on at the start of the document

Hi

Please take a look here:
https://ecoshopn.com/sitemap_index.xml

I get this error message:
This page contains the following errors:
error on line 13 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.

I did the trouble shooting thing in site check. I only enable yoast seo and twitched between 2 themes, Flatsome and twenty twenty but the error doesn’t go away.
Kindly guide me to resolve this issue.

The page I need help with: [log in to see the link]