Thank you for using Apple Support Communities! I understand that you are unable to download apps on your iPhone 6 due to a billing verification error. I would be happy to provide some steps to take next.
If you are unable to change your payment method to None, check out the following resource to learn more:
If you can't choose None when you edit your Apple ID payment information - Apple Support
If there is an unpaid balance due, you might need to update your payment method before being able to update or download apps. The iTunes Store will automatically attempt to charge any outstanding orders to your updated payment method. Learn more about what forms of payments you can use here:
Payment methods that you can use with your Apple ID - Apple Support