Raicheaghau31
You probably have a Swiss Travel Pass, not a SwissPass.
Now why on earth would you want to buy an additional ticket from Luzern to Interlaken, when you already have a ticket for it? Of course, SBB and we Swiss tax payers would cherish that, thanks a lot in advance! 😉
But actually, the Swiss Travel Pass IS already your ticket for this itinerary.