I’m trying to buy gems on clash of clans but I can’t change my payment method to my iTunes card

By iTunes card, do you mean your Gift Card or the payment method you have on file? If the Gift card, have you redeemed it to add to your account balance? Normally, if you have an account balance, funds will be taken from there before your credit card is charged.

Have you redeemed the card to your account balance?

Gift cards redeem.    

Gift Card - can’t redeem.