Hello afk91
First, please try to delete the browser history and cookies on your computer. Then try to buy the Swiss Travel Pass again on sbb.ch under “leisure and holidays” then “international guests”.
If this still doesn’t work, it’s best to call the SBB Contact Center. You can buy the Swiss Travel Pass with a credit card over the phone or directly at a staffed SBB train ticket counter in Switzerland.
I hope helping you and wish you a beautiful trip in Switzerland! 🙂
Kind regard Alban