i just bought a gift card but it said " you must enter a valid code " ?

Hello yousefdj,

I understand that you're unable to redeem an iTunes Gift Card that you purchased online. Fortunately we've an article (linked below) that details a number of steps which can help to get the iTunes Gift Card code successfully redeemed against your account.

If you can't redeem your iTunes Gift Card or code - Apple Support

Sincerely