cant buy in app purchases with itunes gift card credit?

make sure that inapp purchases are turned on... settigs > general > restrictions > inapp purchases = on

if you are getting the contact itunes support error message, then there is a block on your account at the moment, you can call 800.my.apple, or go to expresslane.apple.com select itunes > itunes store > account billing > fill in the requested information hit hit continue, then select chat and chat with an iTunes Advisor