Hello FuriousP
Have you tried if you receive the same error buying your ticket over the sbb website or without loggin in on the app?
Have you already deleted the app once and downloaded it again or switched off your device once?
If you tried all these things and you still cannot buy the ticket, please use the contact form in the SBB Mobile app to contact the support team.
I wish you an excellent afternoon.
Kind regards, RahelW