A customer had a problem with a coupon code.
There are usually a few reasons why a customer might be confused about a coupon code not working:
1- There are conditions on the coupon that they don't understand - perhaps a customer group exclusion, category exclusion, or minimum spend requirement.
2- If the coupons are auto-expiring, they may have already expired by the time the customer tries to use them.
One thing that can help with some of these cases is this open source promo code messages extension. It gives more detailed error messages to customers when coupons are not valid.