Hi I had the same issue but I was able to add a Visa and a Mastercard test card after restarting my test iphone 6s.
Steps to do
create a sandbox use in apple connect
use a fresh email address that never was used on itunes
activate apple Id from email
configure the new device using the test account apple id
add a shipping address to the new apple id
restart the device
go to settings - wallet & Apple Pay
add any of the listed test card details
Enjoy testing :-)