The coupon code box is located on the cart page. But it can be easy to miss!
The cart page is the first page you see when you begin the checkout process.
If you can't find anywhere to enter the discount code then you probably missed it on the cart page and continued to checkout without entering your code.
If you haven't completed your purchase yet and you still want to use your discount code then click the FAW icon on the top left corner of the page to go back to the main site.
Then click the Cart icon in the top right corner to go to the cart page where you can enter your code.