On Luzern - Grindelwald seat reservations are only possible for the Luzern - Interlaken part. They are not possible on Interlaken - Grindelwald.
You will in fact find out that the majority of trains in Switzerland do not take reservations. You should also believe us when we say that even where it is possible it is not necessary.
You can get a reservation for the Luzern - Interlaken train on the Zentralbahn website, but that is in my opinion not worth the money. Just do what everybody does: Go to the station. Board a train. Find a seat.