Here is what you need to know:
- In order to travel on a railway line you need a ticket valid for that route. So tickets are not for a train, they are for transportation. If you are only doing Tirano - Chur you need a ticket Tirano - Chur. If you are travelling beyond Chur. eg. to Zurich you would in that case need a ticket Tirano - Zurich. Tickets are managed nationally, and are fully integrated.
Sometimes if you are purchasing early the system will propose you a Saver Day Pass, if that is cheaper. With the Day Pass you can travel on all public transit in Switzerland, including the Bernina Railway.
On the Bernina Express you can travel using any ticket that is good for that line, so a ticket for a route covering the Berina Railway, or a Day Pass, or a Swiss Travel Pass etc…
If you want to sit in the Panorama cars you also need a reservation for the Panorama cars. This appears to be what you have.
So you have your seat, not you also need a ticket or a pass.