Hello Ellice
The Swiss Travel Pass Flex cannot be added to the SBB Mobile app as you have to activate esch single day before use.
All is explained when you sign in on this web portal:
https://www.activateyourpass.com/
Keep in mind that you must carry your physical passport or ID card with you whenever you ude the STP Flex (no photo, no paper copy of your passport).
Kind regards,
Roland