I just buy the iTunes but the credit redeemed

If you mean that you've bought an iTunes gift card but are getting are getting an 'already redeemed' message when trying to redeem it contact the store that you bought it from, if they can't or won't help try contacting iTunes Support and see what they say (they are likely to want to see images of the front and back of the card and its receipt) : https://www.apple.com/emea/support/itunes/contact.html