Hello,
Thank you for contacting Rank Math, and sorry for any inconvenience caused.
Please set the primary taxonomy in the Titles & Meta Settings of post/page: https://rankmath.com/kb/titles-and-meta/#primary-taxonomy
Enable Advanced Mode in the Rank Math dashboard:

Then go to Rank Math > Titles & Meta > [Post Type] > Primary Taxonomy > Select Categories.

Once done, you will be able to select the primary category from the post:

I hope this helps.
Don’t hesitate to get in touch if you have any other questions.
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.