Tickets are for a route, not a train, so you have a Lugano to Luzern ticket. Now you also need a reservation, and that you can get here: https://www.gotthard-panorama-express.ch/en/
(When booking select “reservation only” under discounts).
Note that what you have is probably a ticket for the train all the way to Luzern, so not the train + boat combination that you want.