Hello JuliaSz
Good day …
Try entering all credit card information in full capital letters.
Using the app, does it work with Google or Apple pay?
Alternatively, you can use Trainline to purchsse tickets, or swissrailways.com (for travel passes).
Unless you want purchasing Saver offers, there is no need to purchase tickets in advance.
Regular tickets are also available the day of travel; Swiss rail tickets do not inlude a seat, thus you will not have any advantage when purchasing them ahead of time.
Kind regards,
Roland