There is no US-based phone number provided. If you have to reach out to SBB, call +41 51 225 78 00.
Your card has to support 3-D secure to make successful payments. 3D-secure is an official security standard used in Europe for webshops. The money has not been deducted but reserved, and will again be back on your account in a few hours.
→ Try to buy your ticket from the Swiss Travel Center instead, there the webshop is less restrictive: Swiss Travel Pass Flex | Switzerland Travel Centre
However, are you sure the Swiss Travel Pass Flex is the right ticket for you? In my opinion, the Swiss Half Fare Card (combined with Saver Day Passes) is the better offer. The Swiss Travel Pass Flex only grants free travel on the activated travel days; on all other days, you will have to pay full fare tickets. With the Swiss Half Fare Card you can get tickets at reduced costs on all days, and you do not even have to spend the money in advance.
Swiss Half Fare Card | Switzerland Travel Centre