Where are you trying to book? And what route? That does not look like an error message one gets on the SBB website. There you should normally always be able to buy a ticket, as the reservation is separate.
Best thing to do is buy a Saver Day Pass, and buy that now. That gives you unlimited travel on all public transit so you can worry about which trains to take later.
If you want to take the Glacier Express you can book that on www.rhb.ch, the operator.