What is the exact text of the message/prompt that you are getting ?
And when are you trying to select 'none' ? You won't be able to do it when trying to download something, you will only potentially get it by just logging into the account : Change or remove your Apple ID payment information in the iTunes Store - Apple Support
If you don't get the 'none' option on the payment details screen : Why can’t I select None when I edit my Apple ID payment information? - Apple Support