Hello,
Thank you for contacting Rank Math support, and sorry for any inconvenience that might have been caused due to that.
Please enable your Role Manager in Rank Math under Rank Math > Dashboard > Modules:

If you don’t see that option, ensure that you have enabled advanced mode to see the option.
After that, go to Rank Math’s Role Manager Settings page and check “On-Page General Settings” option for your Shop Manager:

That should give them access to primary category selection.
Hope that helps and please do not hesitate to let us know if you need my assistance with anything else.
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.