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

A comment when Approving is optional but on Rejecting its mandatory. Upon rejecting, your comment should be clear and concise to inform the original coder why you're rejecting the invoice so they can make the corrections and resubmit for approval.
 
Comments made upon approval or rejection can be seen in the approval history:

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