When you have entered the lessons you can create a timetable. When you activate the item <Scheduling> in the data entry wizard for the first time, you are asked to give a name to the new timetable and to select a time range for it. When you create your first timetable it will always be the entire school year. If you change the timetable during the school year then you could give the new timetable the name e.g. '2018_2019_Sem2' having a different time range, i.e. 04.02.2019 to 07.07.2019.
Click on <Create and open> and the time range of the timetable will be created and opened. Your next step will then be scheduling.