The train in question is an Italian train (even though it is going to use an SBB trainset). So you must book it with Trenitalia. If you were to book it via SBB all that will happen is that SBB will book a ticket with Trenitalia on your behalf.
There are works planned on that line, and for that reason tickets for most fast trains on that route are not yet available. You will need to check again later. It is only May after all.