Before you can generate timetable, your timetable needs to pass the test (menu Timetable/Test).
This test tries to generate small parts of timetable to detect basic mistakes in inputting data or constraints. Finding these mistakes is important, because if it is not possible to generate timetable for just one class or one teacher then it will not be possible to generate whole timetable.
So test can save you lot of time that you will othervise waste on generating "impossible" timetable:
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