Good afternoon @Yanlin.
Thank you for your query.
I also tried to book a trip from Saint Genis to Switzerland on the SBB Mobile App and it worked.
So that we can check the problem, I ask you to send an e-mail with a print screen of the error message to ticketshop@sbb.ch with the following informations: brand and model of the phone and the SBB Mobile version.
I wish you a nice day.
Kind regards.
AferditaS