What is or isn't accepted by iTunes can change over time, whether you've been able to use a card in the past isn't a guarantee that it will always work. By 'Re-entered the number and it said it was invalid' you mean that you've completely removing the card (i.e. the 'none' option showed if you just log into the account) and re-entered it from scratch ?
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).