Bus from Siracusa to Palermo

I'm not having any luck, either, for the same reason. It appears that the bus company is InterBus.

Siracusa and Palermo are both major cities, so I hope you'll find a suitable bus, but buses can be very thin on the ground on Sundays in Sicily. I see that there are three rail connections on November 3:

dep 7:32 AM, arr 2:12 PM (2 changes)
dep 10:20 AM, arr 5:03 PM (1 change)
dep 1:35 PM, arr 6:29 PM (1 change)

The last train of the day cuts through the interior of the island.

I just went to the Interbus site and they show 3 buses for 3 November at 800, 1400 and 1700. Each trip I about 3 hours 20 minutes and costs €13.50.

You can check bus schedules here: Interbus.it.

There are 3 buses running that route on Sunday, November 3; 8:00, 14:00, and 17:00. The journey takes 3 hrs, 21 minutes.

Please note: You can buy your tickets at the kiosk at the bus terminus along Corso Umberto, 196. There’s is no bus station (building)
The buses line up along Corso Umberto and you must keep on eye out for the correct bus.

Enjoy your trip!

The Interbus website mentions that seat reservation is mandatory on the bus from Siracusa to Palermo. This is the text in Italian :
la prenotazione del posto è obbligatoria. Il prezzo del biglietto di corsa semplice é: 11 euro se si acquista su www.buscenter.it, 12 euro se si acquista in biglietteria.
Reservations are to be made on the www.Buscenter.it website, it says, or can be bought in the ticket office.
I just checked on that and November 3rd is already available for reservations on the Buscenter site.
The Buscenter site has a highlighted text saying "The Syracuse Terminal Bus stop has a new temporarily address: CORSO UMBERTO - TRATTO INTERPOSTO TRA LE DUE BRETELLE DI FORO SIRACUSANO (EDICOLA VILLINI)" Seems like the bus stop has temporarily moved. Palermo has 2 destinations : Autostazione and Bar Oreto, with the Autostazione bus stop being the final one.

Thank you so much. That information was really helpful.