It seems that you’ve made changes to the theme and there is an output of an empty character before the initial output.
You can see it in your sitemap that the XML code is starting from line 2 and not from line 1.
I can’t be sure what changes you’ve made to the theme, but from my experience the first files you have to check it header.php and functions.php.
Something you added is adding a blank character before the initial output.
You can try to remove temporary the custom code from functions.php or header.php of your child theme just to locate the issue.