Mahmoud explains Cancelling or Voiding a Purchase Order generated from a Requisition | GP 2013 R2 and later.
Dynamics GP Essentials: Cancelling or Voiding a Purchase Order generated from a Requisition | GP 2013 R2 and later
Sales, Purchasing & InventoryGP 2013
Archived post from 2015. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.