ali_2709
You could only reserve seats on the first IC train to Lausanne anyway. And in this particular train it would only make sense during peak commute times. Even if that IC train you’re boarding at the airport should be quite full, roughly 1/3rd to half of your fellow passengers will leave the train a few minutes later at Zurich HB (main station) anyway, so that would be the perfect moment to quickly change to ‘better’ seats for the rest of the journey.
The reason why you might not see the ‘seat reservation only’ ticket option at the bottom for the selected connection might be that you’re trying WAY too far in advance: Seat reservations are only possible earliest 3 months in advance.
Note that if your incoming flight is delayed, your reservation could not be exchanged or refunded, so the money would be lost.