Hello,
Thank you for contacting Rank Math and sorry for any inconvenience that might have been caused due to that.
The most common cause of this error is caching of the sitemap file. Please check your caching rules and ensure that the sitemap URL is set to NO Cache or bypass cache.
https://rankmath.com/kb/exclude-sitemaps-from-caching/
Looking forward to helping you. Thank you.
I did everything as in the tutorial and it remains the same.https://ibb.co/fqmGZJN
https://ibb.co/t4mSZDq
Hello,
I have checked your sitemap using XML Validator and there are no issues detected with your sitemap 
Can you please follow the these steps and see if that fixes the issue:
1. Flush the Sitemap cache by following this video screencast:
https://i.rankmath.com/pipRDp
2. Clear your website’s cache and remove all the sitemaps from your Google Search Console account, and then submit only the main sitemap (your-domain.com/sitemap_index.xml) (please ensure to replace “your-domain.com” with your actual domain).
Further, give Google some time to crawl your website again and see if the issue is fixed.
Hope that helps.
Thank you.
Hello,
We are super happy that your issue is 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.