Advance purchase train ticket, Salzburg to Vienna

That is because it is an Austrian train and doesn't touch Germany, so Deutsche Bahn does not sell tickets for it. Secondly, it is a private company that is in competition with OEBB. Delta doesn't sell United Airline tickets for the same reason. Look here: https://westbahn.at/en
Reservations are not generally needed as you can buy tickets from the conductor after you've found seats.

Hi,

If you want to leave from Westbahnhof, ie without having to go all the way to Wien Hbf, take Westbahn to Salzburg, very nice trains, I take Westbahn whenever I can in Austria. No need to buy in advance, you pay the conductor on the train.

I'd also suggest trying Westbahn on that route, as their trains are very comfortable. As mentioned you can buy tickets from the Conductor, but one caveat to mention - the portable POS terminals they use don't accept the old fashioned magnetic stripe credit cards so you MUST have a Chip & PIN card. If you don't have an appropriate card, be sure you have adequate cash at hand.

Thanks. Westbahn looks good

DB does happen to sell the national railway (DB and OBB) trains departing from Salzburg, but not the private-brand WestBahn trains that serve your preferred station.