Thank you for contacting support.
That error comes from a constant that should be defined by WordPress by default but on your website is not being defined.
There are 2 ways to fix this issue:
- You can define the constant
- You can disable the caching from the sitemap by using the following filter: https://rankmath.com/kb/exclude-sitemaps-from-caching/#transient-cache
Hope this helps solve your issues.
Don’t hesitate to get in touch if you have any other questions.