Why are you trying to “book” a ticket? You have the Swiss Travel Pass. This is your ticket. There is no need to get another ticket. Given that there is no need for a ticket when you hold a pass there is no option to buy such a ticket (even at zero rate as you seem to expect).
All you need to do is on 18/12 go to the Brig station, find yourself the train you want to take, and get on it. Then when the ticket inspector comes, show them your pass.