Hello Colorado25
For the majority of European webshops, including the one from SBB, you must have 3D Secure ID activated. This is an additional security layer also known as 2FA authentication. Talk to your card issuer to know if your card has it activated.
As for the purchase of the Swiss Half Fare Card, you could also purchase it from the STC, the Switzerland Travel Center, a travel agency run by SBB and Switzerland tourism. Their webshop is less restrictive what concerns the acceptance of credit cards. However, they do not sell other tickets.
https://switzerlandtravelcentre.com/en/che/ticket/swiss-half-fare-card
Kind regards,
Roland