Hello Maryam_mzr
Thank you for your query.
As this is an international trip, the purchase can only be made on www.sbb.ch. We have just tried this and received a price which we can also buy. Please try again. If the purchase still does not work, please contact the Contact Centre on 0848 44 66 88 (CHF 0.08/min) or an SBB ticket office.
I wish you a nice day.
Kind regards, AferditaH