Hi
If you are not currently able to choose None as a payment method, even for downloading free apps and updating existing ones, the following support article explains why that is the case:
Why can’t I select None when I edit my Apple ID payment information? - Apple Support
Follow the instructions here as appropriate:
- Pay an unpaid balance in the iTunes Store - Apple Support
- Change or remove your Apple ID payment information - Apple Support
- Payment methods you can use in the iTunes Store, App Store, and iBooks Store - Apple Support
If you still need help, contact iTunes Support: