Hello,
Thank you for contacting Rank Math and bringing your concern to our attention.
Please note that the star ratings can only be added to a specific set of Schema types so this would depend on the type of Schema you are using for your website.
You can see from this tutorial how to add star ratings on your website with the help of Rank Math: https://rankmath.com/kb/earning-star-ratings-in-serps/
Also, if you have a Google Business profile page with ratings and reviews already, and they are linked to your website, the reviews will eventually show together within your website on the search results pages.
It’s not that you can’t add the star ratings based on the GMB reviews you have on your profile, but that goes against Google guidelines and as such, it’s not recommended to do so because you could incur a manual penalty. Google guidelines also state that the reviews should be generated from users only:

Here’s a link for more information:
https://developers.google.com/search/docs/advanced/structured-data/review-snippet#technical-guidelines
Hope that helps.
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.