I am trying to book the bus from Siena to Florence for Tuesday 17 October. When I choose that date a bunch of options come up, but when I go to buy tickets I get " can't find any tickets for that day" no matter which bus I choose. Do they not allow advance ticket purchases? Thanks!!
Are you using the official site https://shop.at-bus.it/auth/login or the AT App?
If you are planning to take the 131R bus, there is no need to purchase advance tickets. The bus runs almost hourly and if you are departing from Piazza Gramsci there is a Tabacchi shop in the Piazza where you can buy the ticket or the bus ticket office in the underground passage at the Piazza.
Dario, yes both that and Rome2Rio come up with "no tickets for that day" response.
I don't think you'll need to book on that line.