If you're trying to apply a discount code to your order, you will need to do so on the payment page at the very end of the check-out process. The payment page comes after the customer/shipping information page.
On the payment page, the discount box is just above the empty fields you fill out to enter your payment information. Enter your discount code into the empty field in the discount box and click APPLY, just to the right of the box itself. This will visibly adjust the price and confirm that 'You have applied a discount with a value of ___.' Now, all you have to do is to complete the purchase and confirm the order.
If you receive an error message, please take a moment to identify which error you have received. If it says 'Unable to find a valid discount matching the code entered,' please ensure that you have entered the discount code exactly as you received it. If you are sure you've entered the correct code but still receive this error message, please submit a support ticket request by e-mailing us at firstname.lastname@example.org.
If the error message says 'The checkout does not meet the requirements set by the shop for this discount,' this means that the discount code you've entered does not apply to any of the items in your cart. If you receive this error message, please confirm that you have the correct contents in your shopping cart. If you are sure that you do, and are still receiving this error message, please submit a support ticket request by e-mailing us at email@example.com.