Hello kiran potnis
Welcome to the SBB Community!
Because with the Swiss Travel Pass you do not have to purchase a ticket for this journey!
The Swiss Travel Pass is your ticket
What else would you need? A seat reservation? Read this:
https://community.sbb.ch/blog/23887-demystifying-seat-reservations-on-swiss-trains-a-comprehensive-guide
Simply get on the desired public train, take a seat in the respective travel class, and show the ticket’s QR code together with your physical passport (no copies) when the ticket inspector comes along.
If you hold a 2nd class Swiss Travel Pass and you want to ride in 1st class, you can purchase a class upgrade ticket at ½ fare. How to set the half fare option in the SBB Mobile app we described here:
https://community.sbb.ch/d/26418-how-can-i-save-the-info-of-swiss-half-fare-card/2
All clear?
Friendly advice: Saying hi before asking a question to a Swiss, is appreciated. The Swiss consider this polite.
Kind regards,
Roland