Hello GT Sailor
Please note that there are various purchase processes on sbb.ch and that the technical system landscape differs greatly depending on the purchase process. Your described case for the purchase process for the Swiss Halfe Fare Card is not the same purchase process as the international purchase process (Zurich - Milan) described above by Yeegoopouss87.
In 99% of the error messages mentioned, the cause lies in the device and / or browser settings (especially the time and cookie settings).
Kind regards
Nicole