where do i get an Ecard serial number from

I have recently purchased an apple E card for my friend and when they have come to redeem it the error message comes up that its already been cashed in. Upon asking apple support they say they need the serial number off the card but its a virtual card with the redemption code on the screen and no serial number as in a proper shop bought card.

I am trying to make them understand but they say i must contact the retailer for the serial number.

Will it be something they have for virtual cards. Whats the way forward.?