Reservations are totally separate from your pass/travel ticket.
When you make a reservation it is simply that, you have reserved a seat on a train that you have a separate ticket for (Your pass)
Your pass is where you register the train you are going to travel on. This does not make a reservation so if a reservation is needed you do it separately before travel, usually online or at a station in the relevant country.
To create your travel ticket , you add any trains you intend to travel on to the section of your app called my trip. This can be done after you add a pass but before you activate your pass. (There is only one trip per pass - it covers all journeys taken during your validity period)
Near your first departure you activate the pass and, ideally just before boarding you then add your first day trains from your trip (or manually if the train is not in your trip). That generates a QR code that is your ticket to travel for all trains you add to the pass for the full day. Extra trains can easily be added to your pass as you travel
So if you do not travel on a train you have a reservation you simply lose the fee paid. Just delete the train from the pass and add any alternatives you travel on that day.
HOWEVER please note that you cannot cancel a travel day after 23.59 the previous day, hence the advice to only activate your pass on your first day of travel and only add trains shortly before boarding. If you remove all trains on a created travel day in your pass after 23.59 the previous day you will be unable to delete the travel day and therefore lose it.