Hello,
Thank you for contacting Rank Math, and sorry for any inconvenience caused.
Please ensure that Advanced Mode is enabled in Rank Math dashboard:

Then, go to Rank Math > Titles & Meta > Categories/Tags > Turn on Category Archives Robots Meta > Check noindex checkbox > Save

I hope this helps. Looking forward to helping you.
Yes! Thank you! It all made perfect sense after I turned on the advanced mode.
Hello,
We are super happy that we have addressed your concern. If you have any other questions in the future, know that we are here to help you.
If you don’t mind us 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 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.