Q: Change the payment

Have you read my reply on your duplicate thread : I want to change from vise to none can you assist me please

Do you currently have a credit card's details entered on your account, or have you not used the account before and not entered payment details ?

If you have a card entered then do you owe iTunes anything and/or have any payments due (pre-orders, auto-renewing subscriptions) : Why can’t I select None when I edit my Apple ID payment information?

If this is the first time that you are trying to use the account to download something then how did you create the account ? Unless the instructions on this page are followed when creating an account : http://support.apple.com/kb/HT2534

then credit card details will need to be entered before the account can be used to download any item from the store.