Unfortunately, promotional/coupon codes can't be added after an order has been placed or applied to a previous purchase. In order to receive the discount, promo codes need to be added at the cart page or during the checkout process. Please refer to Where do I apply a Promo Code to my order?
As long as the coupon is still valid, it can be applied to a future purchase. If you have any questions regarding your promo code, please contact our Customer Service team at firstname.lastname@example.org