Pass and reservations are completely separate. Reservations are not visible in the Rail planner app, except if you book a reservation via Interrail (not recommended) and log into your account in the app.
If you see "no trains available”, then chances are the timetable isn't complete yet. For more details, please mention route, date and departure time. SNCF don't believe in publishing timetables a long time in advance so you'll probably have to wait. Check this page if you want to know when reservations will open for SNCF trains.
If you want to know why that particular journey is €108, please mention route, date and departure time. It sounds though as if it's a nonsense route with multiple trains that already happen to be in the app at this time. The Rail planner app should actually not be used to plan because it's often not up to date and generally unreliable. The SNCF website will always be most up to date for France, but before reservations are opened, timetables are often not complete.
There's however no need to add trains in the app a long time in advance. The only requirement is that you do it before you board (or before check-in for Eurostar).
For the best ways to book pass reservations, check this page:
https://www.seat61.com/interrail-and-eurail-reservations.htm
Thank you that was very helpful. 