Gift card transaction fail. How to check balance?

Hi. Im very sad and concern trying to figure out this situation my son is been sharing gifts card $100 $150 to someone in his contacts for some games and its been happening alot the person that’s talking to him as a friend told him the card didn’t work That transaction fail My son is 14 years old

and i need to check the balance of this gift card and i dont know how 😞

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