Hello dsmarsh03
I am unsure if you are the person that asked the same question on TripAdvisor, too, which I answered there.
The SBB Ticketshop requires your credit card to support 3D Secure ID. If you do not have such a credit card, you can buy the Swiss Travel Pass from the Switzerland Travel Centre, a partner agency of SBB: https://shop.switzerlandtravelcentre.com
The only disadvantage when buying from STC, your STP will not appear in the SBB Mobile app.
Best regards,
Roland