Hello,
Thank you for contacting us and sorry for any inconvenience that might have been caused due to that.
Please head over to your WordPress Dashboard > Rank Math > Status & Tools > Post: Categories and from there, make sure the Category Archives Robots Meta is set to index:

After that, please check the Advance Robots meta of that category page and make sure this is also set to index:

Please note that some options are only available in Advance Mode. If you are not able to find the Advance Robots Meta tab, please try to switch to the Advance Mode: https://rankmath.com/kb/advanced-mode/
Let us know how it goes. Looking forward to helping you.
Thank you.
Hello Rakibuzzaman Siam.
Thanl you very much for your quick reply!
Hello,
We are super happy that this resolved your issue. If you have any other questions in the future, know that we are here to help you.
If you don’t mind me asking, could you please leave us a review (if you haven’t already) on https://wordpress.org/support/plugin/seo-by-rank-math/reviews/#new-post about your overall experience with Rank Math? We appreciate your time and patience.
If you do have another question in the future, please feel free to create a new forum topic, and it will be our pleasure to assist you again.
Thank you.