If you are using a discount code in the checkout and it wont go through,these are the most common reasons why.
1. Your overall order before shipping is below USD$40 (if using an affiliate code)
2. The code you are using has been used before or is invalid.
3. The email address you are using is not the one used for your account (if you are using rewards program points)
4. You have used one discount code already for this purchase. Only one discount code can be used at a time. E.G. If you are trying to use a general discount code and a rewards discount code.