How to use a code at checkout, and what to do when it does not work.
You need an account, and your email and phone must be verified. If you opened a coupon link, the code is already filled in for you.
The cart changed (for example it fell under the minimum or has sale items), or the code ended.
Coupons need a verified email and phone number. You can do this in your account settings.
No. Remove the sale items to use a code. See Sales and cashback events.