Hello,
Thank you for contacting Rank Math and bringing your concern to our attention. I’m sorry for any inconvenience this issue may have caused you.
Could you please share the affected sitemap URL so we can check? You can add it in the sensitive data section upon editing your first post

It is completely secure, and only our support staff has access to that section.
Meanwhile, the error message you are seeing indicates that there is some extra content before the XML declaration of your sitemap file. This could be caused by a plugin, theme, or server configuration issue that is adding whitespace, comments, or other output before the sitemap is generated. Most likely a plugin has added blank whitespace at the beginning or end of its code.
You can check this link for more information:
https://rankmath.com/kb/fix-sitemap-issues/#xml-text-declaration
Looking forward to helping you.
Thank you.
		
	 
	
	
	
	
 
			
				
	
	
		
		Thanks for your message, I’ve been able to do it 
		
	 
	
	
	
	
 
			
				
	
	
		
		Hello,

You’re welcome.
We are super happy that this issue is already resolved. If you have any other questions in the future, know that we are here to help you.

If you don’t mind me asking, could you please leave us a review (if you haven’t already) on https://wordpress.org/support/plugin/seo-by-rank-math/reviews/#new-post about your overall experience with Rank Math? We appreciate your time and patience.

If you do have another question in the future, please feel free to create a new forum topic, and it will be our pleasure to assist you again.

Thank you.