Q: iTunes is saying my iTunes voucher hasn't been activated properly?

Assuming that you don't know where it was bought from and don't have the receipt (so can't take it back to the store and ask them to activate it properly) 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 if you haveit 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