Hi Iris,
to Zadar are no trains for some years already, unfortunately… you have to take a bus from Zagreb, Sibenik or maybe Knin (not sure if there are buses from Knin). Bus is not included in interrail and you need to buy a separate ticket.
In other cases, if there is really a train (on other routes) and it is not shown in the app, then you can add it manually .
Safe trip
You have to take a train in direction of Split till Knin from there goes a bus to Zadar last year ot still was a rail replacementbus and because of that covered by EURAIL Just add the trips manually please note that you need a reservation on the train to Knin