My app and this site insists there is an airport bus going from Schottentor/Liechtensteinstrasse to the airport. I have been at Schottentor many times. I am unable to find either bus or Liechtensteinstrasse. I am severely disabled and cannot walk very far, so I would appreciate very precise instructions.
The zoomable map at https://www.viennaairportlines.at/en/stops shows Liechtensteinstraße and the airport bus stop, 3-4 blocks from the tram terminal at Schottentor. The "stop info" says
Vienna Airport Lines buses stop on Liechtensteinstraße (by the Liechtensteinstraße exit from Schottentor underground station, on the U2 line).
which suggests that there is a separate eastern exit from the U-Bahn station.
The map at https://www.wien.gv.at/stadtplan/en/grafik.aspx?lang=en&bookmark=jildRsI0NEZJg6NErTsURO5RphlRnKnmkev2pn4Mpr4C appears to show the route of the airport bus as a green line. It agrees with the above map about the location, but on the other hand shows U2 one street over from where it looks like it is on Google Maps (and the few street photos I can find), so caveat lector.
(I have no personal experience with this location, but it grew too long for a comment).