In some cases, the promo code may not be applicable. In other cases, it could have expired or have been entered incorrectly. Let users know about it by providing an explicit error message, including constructive advice on what users can do about it. If the promo code has expired, you can provide a link where users can browse through the list of other available coupons. If the promo code contains an error, suggest that users check and reapply it.