When a review request has been completed (approved or declined, but closed anyway), the comment box still appears in the request as if the comments are allowed.

However, when one attempts to send a comment, they would get a toast with the error (see screenshot).

It would be a more intuitive experience, if the comment box was removed or disabled for the completed reviews in the first place, instead of allowing the user to input the comment - to only find out they cannot send a comment.