Sorry to inform you that coupons are not redeemable on discount items listed on our store, as they are both promotions and this does not comply with our policy. You are able to add the coupon code to all other items listed except for the items using the tag "Promotions" or in the "Promotions" folder.
P.S. There is a how-to video and images listed on our solutions page for Online Store in our knowledge base.