Because of engineering works the schedule has been changed (as the SBB planner actually tells you). So you take another train in stead. For example the departure at 09:09
The thing to remember is this: Tickets are for a route, not a train. You may believe that you bought a ticket for the 9:10 to Luzern, but you actually bought a ticket for transportation from Luzern to Alpnachstad. With supersaver tickets you get a discount if you commit to taking a particular departure, but that restriction is lifted if due to a schedule change that train does not run.