None of the trains on Chur - Tirano will be fully booked as trains in Switzerland aren’t booked. You can always buy a ticket Chur - Tirano, even just a minute before departure.
See www.sbb.ch where you can do that. In fact you can book Zurich - Milano via Tirano there. You can also just get that at a station.
If you want to sit in the panoramic cars that are attached to some of the trains you do need a reservation, but that is not needed for the rest of the train. The reservation you can get on www.rhb.ch, but it does indeed look like the direct Chur - Tirano train has no free seats left in the panoramic cars on that day. You can of course still travel in the normal cars, or on other trains.