If you can’t get the app to display a journey then you can add it manually but only if it is a valid journey.

For a reasonable idea of which operators you can use look on app under More - Rail network Guides.

You should not rely on the app having fully accurate schedules, it runs offline and is not updated as often as other sources.

Use websites of local operators and better European operators that have up to date schedules available, such as https://www.bahn.com/en

Reservations are separate from the app/pass. For all trains with compulsory reservations and any others that you may wish to get reservations for you will have to search for them on interrail site reservations section or other external sources, many of which are explained here:

Buses and ferries are not normally included in interrail global pass. There are some exceptions but best assume they are not included unless you are told otherwise.

Some ferry operators offer discounts for pass holders, this is not part of pass validity using a travel day but an additional benefit, details are on interrail website or in app under More - Benefits.

Jadrolinija Zadar - Ancona is not one that has any interrail discount.

SNAV Split - Ancona is listed as 20% discount.

Ferry companies offering a discount can be found on the Benefits portal:

https://benefitsportal.eurail.com/

When you only get a discount, then do not enter the ferry into the app because you do not need to use a travel day.