You can enter a comment on the PO before you approve or reject it. You’ll find the comment field just over the approve/reject button.

Entering a comment is optional when you approve the PO, but a comment is mandatory when you reject the PO. Upon rejecting, your comment should be clear and concise to provide the original coder with the reasons you're rejecting the PO so they can make the corrections and resubmit for approval.
Comments made upon approval or rejection can be seen in the approval history: