If you fail an exam, you may schedule another attempt. Do not reschedule the same appoinment; create a new appointment for the additional attempt.
Your subscription does have a limited number of attempts, so you may only schedule another appointment if there are attempts remaining on your subscription. Additionally, you may only attempt a given exam 3 times. If you have not passed the exam after 3 attempts, you will not be permitted to schedule it again, until a new version is published.