Seat reservations can be made at the earliest 60 days out. It might be that the carriage has not yet been planned.
There is no need for a seat reservation for this train from Zürich HB to Chur in 1st class for individual travelers; there will be enough free empty seats in 1st class. On a Sunday afternoon, the trains from Chur to Zurich HB are fully booked as people return from mountain excursions, yet not in the other direction.
Concerning seat reservations, for national services, these are optional and only possible on trains of category IC or PE. On Swiss trains, we generally have a free seating system.
For more travel advice, see our English-speaking community: SBB (EN) - SBB CFF FFS Community