“Not in pass network” means that a train is operated by a company that doesn’t accept Eurail passes. 

I think in this case it’s the fast ferry that’s causing problems. You can just buy a ticket for the ferry at the port if you choose to travel via that route.

If you take the direct ICN, where the train itself travels on the ferry, you don’t have to pay separately.

I was trying to book the direct ICN.

On the Interrail Planner, I have selected the trains; saved the journey so they show on My Trip. 

When I select Book Seats / it flips back to the How to Book Seats screen -    or circles . ..

Solution / stop using mobile;  used PC and result first time - even a civilised travel time of 10 something instead of 7. 

Perfect!