A customer/member tries to use a coupon that is only good for full-price items to purchase an item that is on sale. She becomes confused when you explain that she can't use the coupon.

If a customer does not understand why she cannot use a coupon for full-price items to buy a product that is already on sale, you should show her the details stated on the coupon. Such information about the conditions of use of the coupon is usually stated in the lower corner. If the customer understands the situation better, you can add that, by buying the product on sale, she is still saving money, and can use the coupon on other full-price items, saving even more.

The item on sale already has a discount

Explanation:

Usually coupons offer discount on items a store sell but they have indications that state that they are only valid for full-price items. If you want to buy a item that is on sale, the coupon won't be valid as the item already has a discount included and the store won't give an additional discount on that product.