It's possible that your GO Points gift card has already been redeemed onto another account. When a GO Points gift card code is entered, it's automatically redeemed onto the account the user has signed in on. You may also want to consider if friends or family members had access to this gift card code, or if you have any other accounts that might have been used to redeem this gift card.
GO Points gift cards can't be transferred to another account once the code has been redeemed.