It appears you may have failed to read what was said to you when you received a Gift Code.
You must complete Episode 1 before processing any code you may have received. When you have successfully completed Episode 1, try using "Player>Rewards>Gift Codes" again.
Thanks for reading.
Gift Codes can be received the following ways:
To use your Gift Code...
On Desktop: Go to "Player>Rewards>Gift Code"
On Mobile: Go to "Player>Gift Code"
Then enter the code in the input box and click "Send Code".
Gift Codes always have an expiration date, often after the month they're added. Furthermore, there is a limitation to how many accounts can process them, so be quick about using it if you want to claim your reward!