Note: You are currently viewing documentation for Moodle 1.9. Up-to-date documentation for the latest stable version is available here: error/forum/nopostforum.
You need to enrol in the course in order to post in a forum. You may do so by returning to the course page and following the "Enrol me in this course" link in the administration block.
If you are already enroled in the course, you may not have permission to post to this particular forum, as set by the Teacher for the course or the system administrator.