I\'m not able to login into my new iPhone 6s(iOS 10.0.1) with this sandbox account for testing apple way. While able to login into web icloud, itunes and tested on my old iPhone
I have tried this many time. And getting same thing as you described. At last i were able to add test credit card for testing apple pay by following these steps:
To get test credit cards details: You can check the cards details shown in this document: https://developer.apple.com/support/apple-pay-sandbox/
Note: When you add test credit cards, some credit cards detail might get failed in verification, then you have to try another one, Luckily you will be able to add after 3-4 tries.