You made reference to this question: SwissPass Card Delivery, it’s about the Half Fare Card on SwissPass.
First of all, you are not required to have the Half Fare Card in order to purchase half fare tickets. Neither the online ticketing system nor the ticket agent will ever verify whether it is permissible to purchase half fare tickets. This is the sole responsibility of the ticket inspector on board a public transportation vehicle (bus, tram, train, etc.).
That said, when you purchase the Half Fare Card online, you will receive a temporary paper with a QR code, which you can download and use as the Half Fare Card until the red SwissPass card is produced and delivered to your location. For my part, I do not see any advantage of purchasing the Half Fare Card online. You are welcome to purchase your half fare tickets now, and the Half Fare Card on SwissPass at a later stage, for example once you are in Switzerland - would work as well! In this case, the ticket agent will simply issue a temporary card, valid as long as you have not received the red card. In both instances, the temporary Half Fare Card is solely valid with your valid physical passport.
Once you have received the red card, you will be able to register for SwissPass Mobile. The error message currently displayed on your phone will be gone. Just get back to us if you need help in setting up SwissPass Mobile.
To buy the Half Fare Card, you need your passport and a resident address in Germany (or Switzerland), and a passport photo, either physically available or as digital version.
→ All important information on the purchase of a Half Fare Card on SwissPass (no matter if online or from the SBB Travel Center) you can find here: Help for Half Fare Travelcard | SBB help & contact
Please let us know if you have further questions.