The reason you are getting this message is because the gift code has already been used. Once gift code is loaded it should show up on checkout page next to your payment method on the left side of the page which you can then apply towards your purchase.
Gift code is considered "used" once it has been loaded into your account. This is simply because your gift code amount is now transformed into cash and ready to be used at the time of purchase.
If you still have issues please reach out to us at firstname.lastname@example.org or via chat for immediate assistance.