I assume that is 19:38.
Reservations are not necessary, and are anyway not possible for most trains. Just get on, find yourself a free seat, and travel.
BTW. You would have paid the same price if you had bought your ticket on 2024.03.06 at 19:10 at the airport… Not only are reservations not needed in Switzerland, buying train tickets half a year in advance is not needed either. This is mass transit. Like taking a subway, but on nicer trains and with scenery.
A normal point-to-point ticket is good for all trains on the route, so if you happen to be able to take an earlier train, just do so. You do not have a ticket for the 19:38, you have a ticket for “Zurich Airport to Pontresina”.
Note that when buying so far in advance the system should have offered you a Saver Day Pass, which would have been much cheaper.