If the requisition is also committed to a Project\Cost Category it cannot be cancelled, deleted or Voided. This is noted in our new feature blog and new feature documentation for GP 2016 R2. As of this time, this is not a feature that we have available in GP. Users would like to have this ability so that they are not stuck with an Open PO that they do not want to receive against.

https://community.dynamics.com/gp/b/dynamicsgp/archive/2016/12/14/purchase-order-processing-cancel-purchase-order-when-linked-to-a-requisition-amp-visual-indicator-if-quantities-not-fulfilled