Q: Al canjear una tarjeta me dice que el codigo no es valido

Was the iTunes card issued in the same country as your account is based (they are only valid in their country of issue), and it's an iTunes or App Store card, not an Apple Store card (Apple store cards can only be used in Apple stores and their online store) ? If it is and this page doesn't help then you will need to try contacting iTunes Support (you will probably need to give them images of the front and back of the card, and possibly its receipt) : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then iTunes Cards And Codes