Q: I cannot change my app purchase to none..so i must pay for applications free

You won't be charged for downloading items that are free to download (e.g. apps that have 'get' instead of a price button), not for downloading updates to apps.

it's an existing account that you currently have your payment details on, and you are trying to remove the card's details just by logging into the account (not when trying to download something) ? There are some reasons on this page why 'none' may not appear : Why can’t I select None when I edit my Apple ID payment information? - Apple Support