Yes, you can cancel the polygraph. And if it is in relation to a criminal investigation, you most definitely should cancel it. As used by police, the polygraph often serves as little more than a pretext for interrogating a suspect without a lawyer present. Polygraphy has no scientific basis, is inherently biased against the truthful, and the interrogation tactics used can be quite coercive. As a common courtesy, however, you might want to call to cancel the polygraph first thing in the morning, rather than waiting until an hour before the appointment.

See the following video presentations on why, in general, it is unwise to speak with police:

In that case, you really should consult with a lawyer regarding your rights and obligations under the terms of your probation.