Hello SunnyHB
A train ticket and a seat reservation are two separate things in Switzerland. A ticket is mandatory to take a public transport service, a seat reservation is optional yet often not possible. Only seats on IC, PE, EC, RJX, and TGV trains can be reserved.
You can read more on seat reservation in our blog:
https://community.sbb.ch/blog/23887-demystifying-seat-reservations-on-swiss-trains-a-comprehensive-guide
To make an optional seat reservation on the PE Luzern Interlaken Express, see this website:
https://store.luzern.com/luzern_en/specials/daytrips/zentralbahn-int-lu
Book a seat on the right-hand side for better views.
Kind regards,
Roland