With the Coupon Code Error Messages extension for Magento® you can create, track and display custom error messages for customers when coupon codes aren't working upon checkout through your Magento® store. The extension displays a custom message that provides the reason why the coupon is not valid. It includes error messages for coupons that are past their expiration date, coupons that have already been used once, and error messages when a customer’s login is required.
The Coupon Code Error Messages extension saves customer frustration by providing a reason why a coupon code has been refused by your eCommerce store. This can create a better user experience for your customers. It can also reduce time your customer service team spends answering the same questions from frustrated customers who try to apply invalid coupon codes at checkout.
More info at https://www.cminds.com/ecommerce-extensions-store/custom-coupons-error-messages-extension