Why can't I post in a particular thread?

Each thread carries its own read and write permission, set by the club's admins against the role ladder. An announcements thread is typically readable by everyone and writable only by admins; some threads are members-only to read at all.

So the likely reasons are: you are still a pending member, your role is below what that thread requires, or the thread has been unpublished. None of these is something support can change — ask the club's admins.