From Lucerne to Rijeka there is daily bus connection during main summer season, out side of this period there is 3-4 weekly departures. The travel time for the 720 km long bus routes is about 11 hours. In Lucerne the bus departures from the bus station Inseliquai which is situated in the centre of town,, in Rijeka passenger will be dropped off at the main bus station, from where there are connecting buses to most parts of Croatia. If you plan to return from Rijeka to Lucerne, you should buy a return ticket, as you can see below return tickets are much cheaper then two single tickets. (return timetable you can see here)