From Yoast
We looked at your sitemap and we see something in your WordPress install, either a plugin or your currently active theme is outputting whitespace (see the attachment screenshot), and it shouldn’t output white space. Please, refer to this document for more information about the issue: Sitemap error: XML declaration allowed only at the start.
The blank space or line issue could occur due to a number of potential factors (there is a blank line before or after the closing PHP tag in your currently active theme or 3rd party plugins code is most common). In this case, you’ll need to identify the relevant blank line from your theme or contact the theme developer assistance and having them fix it for you. There should not be an empty line before or after the PHP closing tag.
In addition, you can switch your theme temporarily to the WordPress default 2017 theme to see if this resolves the issue (which should). If this resolves the issue, you’d need to reach out your theme developer for assistance to find out why the theme is showing the blank line or space before the XML sitemaps get starts.
Additionally, you can perform a conflict check. This will narrow down the conflict to a specific plugin or a theme conflict. If you are unfamiliar with checking for conflicts, we wrote a small guide that will walk you through the steps. You can find it here: How to Check For Plugin Conflicts.
Please let us know if you’ve any further questions
