It looks like you're new here. If you want to get involved, click one of these buttons!
Posting an answer to this "problem" because I couldn't find it in this forum, but I just figured out what's going on. Back in Vanilla 2.0-2.2, there is a thread on this topic. The solution was unchecking the box "Display root categories as headings". I didn't know what this meant. Well, this is what it means:
To keep a busy forum neat and organized, instead of allowing anyone create a new top-level discussion, the administrator can keep the top level clean by only allowing new discussions in sub-categories. The root category then becomes a heading (and its associated description) for the discussion started by the admin.
Let's say you want a category called Introduction. An introduction is just that. People can comment under Introduction provided there is a sub-category or a discussion already started. To start a discussion under the root category, you have to uncheck that box, refresh, start the discussion (let's say the discussion is "Introduce Yourself") and then go back and recheck the box so that users can't create new discussions under the root level Introduction.