Hello Peter_Chang
I should have added, the Swiss Travel Pass Flex will not be available on the SBB Mobile app at all. You can use the latter to purchase tickets for the days on which you do not use your STPF.
Make sure you activate each day of your travel pass for the day of travel, latest before boarding a public transit service on ehich the STPF grants free travel.
To activate each day of your pass, go to
https://www.activateyourpass.com/
Kind regards,
Roland