What will happen if I cannot make my appointment? Does it count as an attempt?

Yes, it counts as an attempt.  If you do not keep your appointment and you do not cancel or re-schedule it prior to the start date and time, then your appointment will be regarded as an attempt. The proctor will update the status of the appointment to "No Show" and this will be reflected in the Certification Hub under Exam Appointments | Previous Appointments. 

Can I re-schedule an exam appointment?

Yes, you can re-schedule an exam appointment prior to its start date and time. There is no penalty for re-scheduling and a re-scheduled appointment does not count as an attempt. You can do this from the Exam Appointments | Upcoming Appointments section in the Certification Hub.  Use the calendar icon to re-schedule an appointment.  Do not re-schedule an appointment associated with an incomplete attempt.