Frogringer
You’re holding the Swiss Travel Pass, not the SwissPass (the latter being a NFC card with foto ID designed for long-term passes for local residents)
Junfrau Railways is a special case where the STP discount is 25% (not 20%) instead of the usual 50%, therefore that particular discount is not supported on the SBB online sales channels.
However it works on the Jungfrau Railways Website www.jungfrau.ch, where the STP can be selected as discount option in the 2nd “configure ticket” step of the purchase process.
Of course it will also work at staffed ticket desks e.g. at Grindelwald Terminal.