Installed Samsung Pay from the Play Store on Galaxy S6 but had problems validating my American Express credit card. When I tried to activate, I got a message that a validation code had been sent to my email and I needed to enter that code into the Samsung Pay app to get validated. However, it took so long for the code email to arrive that I think the app timed out and gave me a message that the code was invalid. This repeated 5 or 6 times and eventually I was told to validate by calling American Express. This worked.