Why can't I start new topics in some forums here?

Maybe you got the answer already since this thread is 7 days old, but you should select one of the sub-categories first to create a new topic. The sub-categories are the drama/show titles.

Typically, it would make more sense to go to a drama/show page (instead of forum) and clicking a discussion section from there to create a new topic, unless the drama/show you want to discuss is super popular that it appears on the first few page of the sub forum.