What is the exact text of the message that you are getting when being prompted to enter card details, and the error message that you then get ? And do you get the 'none' option just when logging into the account, not when trying to download something ? If this is the first time that you are trying to download something (and you didn't use these instructions when creating the account : Create an iTunes Store, App Store, or iBooks Store account without a credit card or other payment method - Apple Support) then called card details will need to be entered before the account can be used.
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 and your iTunes account are - not all debit cards seem to still be accepted.