Double-check the coupon's expiration date and ensure it applies to the items in the cart. Verify the coupon code is entered correctly, without any typos or extra spaces. Check the terms and conditions of the coupon to see if there are any restrictions or minimum purchase requirements. Contact customer service for assistance if the coupon still doesn't work. Try using a different browser or clearing the browser's cache.
If none of these steps resolve the issue, it might be best to look for another valid coupon code.