These hosts have no interest in you. Only your booking and receiving your money. They have listed their apartment as being at Karlzplatz. It’s a 15 minute walk from Karlzplatz. However the metro station is within a couple of minutes walk of the apartment The station is Taubstummengasse. It is the next stop after Karlzplatz on the U1 line (red). Ensure you arrive at the precise check in time given because if you are not, your hosts will happily abandon you in the street whilst their caretaker checks in other guests at their other properties, leaving you in the street for over an hour.