Where is the itunes gift card serial number

I have a gift card from a store that i purchased it states card is active. After putting the code in it says the card has not been properly activated. No answers that say put your code in again please for I have done it numerous times and this is not the first time I bought a gift card. Jut want to know if anyone knows where the gift card serial number is? I click on the question under support and it does not answer it, just takes you back to a page you have already been on. I like apple stuff, but this BS really annoys me that it can not be a simple fix.