I'm 100% sure of that the cashier didn't activate your gift card by the cash desk, what you have to do now is that take your card back to the shop where you bought it and ask them to check that is it activated yet, if it's not activated yet then you can not redeem it. If you didn't take the receipt with you when you bought this gift card don't worry, every single supermarket can trace your purchase. If your card is not activated yet they will give you a new card.
There's quite often some issues when the cashier is activating the gift cards.
Let me know if this helps.