XML Parsing Error: XML or text declaration not at start of entity Location:

Hello,

Thanks for contacting us and sorry for any inconvenience that might have been caused due to that.

This error suggests that there must be some white spaces before the XML Sitemap’s source code.

This might be added by a blank line/space in your wp-config.php file or your theme’s functions.php file. If there is no such blank space in your wp-config.php or functions.php file, check for a plugin conflict using this guide: https://rankmath.com/kb/check-plugin-conflicts

Also, make sure your cache is disabled as it is highly likely that the cache is interfering with the working of Sitemap functionality of Rank Math.
https://rankmath.com/kb/exclude-sitemaps-from-caching/

Hope this helps. Let us know if you need any further assistance.