Please try the following:
▪ Enter your email address first, then apply the coupon code
▪ Check the coupon code has not expired
▪ Check the coupon code has not been used in your past purchase(s)
▪ If the coupon has limited no. of uses, it may have been fully redeemed.
▪ Check the coupon code is applicable for the products in your cart
Do note that some coupon codes can only be used on one-time purchase products.
If you have autoship ("Auto-renew") products in your cart, such coupons may show an "Unable to find a valid discount matching the code entered" error.
Kindly go back to your cart page, remove all autoships from your cart and check out again.
To speed up the process, kindly have the following ready:
▪ The URL of the page
▪ The coupon code you're using
▪ A screenshot of the error