Hello Quaulleebi82
To buy half fare tickets, the (Swiss) Half Fare Card is not a precondition. You can buy these tickets now and the Half Fare Card later. You must be in the possession of the latter latest when you board a public transport vehicle with half fare tickets. The ticket inspector wants to see both, your ticket and the (Swiss) Half Fare Card.
There is another reason for the unsuccessful purchase: your credit card has to support 3D secure ID in order to purchase from the SBB webshop. Talk to your credit card bank to activate 3D secure ID.
Let us know, should you have further queries.
Kind regards,
Roland