Oct 7, 2014 - 6:51 AM
Seat reservations are not required for trains in Switzerland, expect for a few tourist trains such as the Glacier Express. You can just board any train and pick a seat if you have a pass or a ticket. Your travel dates are in high season though, so if you prefer to book seats, you can do so for trains that allow it. The leg Zurich to Visp can be reserved. Visp to Zermatt normally not. So you can book the first leg if you wish. Most trains from Chur to Zurich can be reserved.
The system rejects your dates because there will be a new timetable for all trains in Europe as of December 14. I expect the new timetable to be released mid October. That should open up your dates for booking. This also applies to Glacier Express seat reservations. The Winter timetable of that train also starts on December 14. Be sure to get the 20% discount!