From other comments explaining the kind of scams running at the moment, one possible scenario is that the card may have been taken, tampered with by a scammer (and the code recorded), and then placed back in the supermarket, with the scammer waiting until the OP purchased it and it was activated at the checkout.

Perhaps between the scammer redeeming it and the poster then trying to redeem by entering the same code, the scammer’s account was flagged and then the OP’s account terminated along with the scammer for using the same code (even though the OP had done nothing wrong).