iTunes billing

See About payment card authorization holds from the App Store or iTunes Store - Apple Support. You need sufficient funds for a nominal charge, typically $1/£1 or the value of any potential subscription if you're tried to set up a free trial.

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When you add a card to an account it has to be a valid card and pass the iTunes validation, you don't have to be buying something at the same time.

For a card to have a chance of being accepted it needs to be registered to the same name and address (including format and spacing etc) that you have on your iTunes account, and have been issued by a bank in the country where you are (and therefore the country which is on your iTunes account).