Hello AshishL
It makes zero sense to book Mt. Titlis tickets in advance. What if the weather is rainy and stormy? We suggest purchasing mountain excursion tickets earliest the day before travel, as the weather is not predictable more than two days out.
You can purchase your Mt. Titlis tickets at the SBB Travel Center in Lucerne train station or even at the machine at the station (ask someone wearing a yellow vest for help).
Kindly note, you have a Swiss Travel Pass; the SwissPass is a resident product. You should not mix up the two.
Kind regards,
Roland