If you have the option to create a post within the GMB UI (with a Posts tab) then your primary category qualifies. I am noticing a delay in posts publishing for categories and listings that have done posts in the past so this may be an unrelated issue.
Drugstore - limited to update or event
Pharmacy - limited to update or event
healthcare and provider related categories have full post capabilities
Casino - no posts feature
Adult entertainment store - no posts feature