Hello,
Thank you for contacting Rank Math and sorry for any inconvenience that might have been caused due to that.
I’ve gone through a few steps and troubleshooting procedures while investigating the issue you’re encountering with. Your sitemap is working perfectly fine and it’s valid. But it seems that all 4 variations ( HTTP, HTTPS, and WWW,non-WWW version of your URL ) of your sitemap are showing HTTP status code 200 and none of them have 301 redirection. I’ve tested them using https://httpstatus.io, and here is a screenshot for your reference:

Here only one of the 4 variations should show a 200 status code. The 3 remaining ones should show 301. Please try redirecting three other variations and it should be fixed.
Exclude the sitemap from caching(If you haven’t) after you apply the fix:
https://rankmath.com/kb/exclude-sitemaps-from-caching/
Also, you can try re-submitting the sitemap to GSC if you see no changes in the error.
If the issue still persists, please try the following URL I’ve shared below and go through each step to resolve the issue:
https://rankmath.com/kb/couldnt-fetch-error-google-search-console/
Let us know how it goes. Looking forward to helping you.
Thank you.
Hello,
Since we did not hear back from you for 15 days, we are assuming that you found the solution. We are closing this support ticket.
If you still need assistance or any other help, please feel free to open a new support ticket, and we will be more than happy to assist.
Thank you.