Double-check the coupon's expiration date and scope of application to ensure it complies with the usage rules. Verify with the website's customer service to see if the coupon code is still valid. Ensure the coupon code is entered correctly, without any typos or extra spaces. Try using the coupon code in a different browser or clear the browser's cache and cookies. If the problem persists, report the issue to the website's support team for further assistance.