Mar 29, 2016 - 7:00 AM
The Bernina Express does not go from Chur to St. Moritz (it goes to Tirano). Chur to St. Moritz is only a small part of the Glacier Express route, so that might be the reason you can't book seats for that leg only. It would be relatively expensive to pay the full surcharge just for that leg. Also you would be limited to only a few trains per day, and only in the afternoon.
There are plenty of normal direct trains from Chur to St. Moritz. Please see the timetable. You don't need to book them in advance and they'll be perfectly fine to enjoy the scenery. It's one of my favorite trips!