Katko When trying to book via SBB, I get this message: Unfortunately, there are no longer any tickets available for the connection you have selected.
That is the unhelpful message the SBB site throws up when it encounters an empty seat inventory for a train. It is unable to distinguish between “train is sold out” and “train is not open for booking yet”. This is the SBB trying to dethrone the DB as champion in the “confusing passengers” department.
Trains do not sell out months in advance. They would be useless if they did.
Just wait a bit. If the train runs it will eventually become bookable, and otherwise you book another train.