Subject 3 can't make an appointment to receive short messages.

First, it may be because the appointment is unsuccessful or not ready. You can make an inquiry according to the business serial number when making an appointment.

Second, it may be that the mobile phone number is wrong, but you have succeeded in both subject 1 and subject 2, which should be impossible.

If it weren't for these two points, you would have to ask the driving school or the vehicle management office.