Q: Why can't I select "none payment" even though my balance is $0

Hello guys.

I am trying to change my region from USA to Vietnam to download some apps. I followed the instruction from the below website and as I checked there was no bill or amount which I owe Apple so why couldn't I change my payment to none. Moreover, I have no subscription as well as automatic membership renewal and family sharing. I appreciated so much for your time guys. Here is the instruction: View, change, or cancel your subscriptions - Apple Support

This is the purchase history which showed that I have $0 balance in my account.

This is account info.

This is payment type in USA

And this is the payment types after I selected Vietnam region. (There was no "none" option)