Since you are using NGINX, add the following code to your NGINX configuration file or you can ask your hosting support to add it.
# START Nginx Rewrites for Rank Math Sitemaps
rewrite ^/sitemap_index.xml$ /index.php?sitemap=1 last;
rewrite ^/([^/]+?)-sitemap([0-9]+)?.xml$ /index.php?sitemap=$1&sitemap_n=$2 last;
# END Nginx Rewrites for Rank Math Sitemaps
Nginx is known to sometimes hinder the generation of sitemaps on a number of site. The above configuration will ensure this doesn’t happen and your sitemap is generated successfully.
Once you have added the code, please check the following:
Is the sitemap working fine in your browser? You can check that by going domain(.)com/sitemap_index.xml
If it is valid, head over to your GSC and submit only your main sitemap file(sitemap_index.xml). If you had submitted any other sitemaps, please make sure to remove them when submitting Rank math sitemap. Let us know if you face any issues.