Hello paschtraveler
The ticket is fine for your journey.
On regular Swiss trains, for example from Zürich to Chur, you do not need a seat reservation. You simply board the train in 1st class and get an unoccupied seat.
Now for the stretch from Chur to St. Moritz, you do not mention if you intend to use the Bernina Express or any regular service. For the Bernina Express, a seat reservation in 1st class is recommended, not so for any regular service. You can reserve a seat with the railway operator directly, which is RhB: https://tickets.rhb.ch/en/pages/bernina-express
Best regards,
Roland