First of all, in the properties of the card catalog on the 'Links' tab, you must indicate that task schedules may be linked to cards from the relevant card catalog.
A user (or user group) then must have or obtain the central right 'Create task schedules'. You can grant this right via Application management> Users and user groups> Central rights.
Users who have this right can then add a task schedule via the Task schedules module or via the details of a card on the 'Tasks' tab.