How come the lecturer cannot schedule a flexible course even though he is qualified to do so?

If the Lecturer is qualified to plan a flexible course as explained in this guide but is unable to do so, it is because the Course he or she is attempting to schedule is set with duration "Set manually or until the end of the Teaching Year."

The function was blocked for these Courses as there would be no limit to the scheduling of classes and the Lecturer could schedule lessons without limit.