The schedule type—group or individual—determines how lessons are opened. It is selected before any students begin learning, and after the initial start, the type is locked to preserve existing access schedules.
Until someone clicks "Start Learning," the "Schedule" tab displays the lesson type switches and the "Apply" button. Once at least one student has started learning, the lesson type selection disappears (a warning appears in the interface), but lesson dates and modes can still be edited within the selected type.
Read more about both types in What's the difference between a group and individual schedule?