I am trying to book a seat on my first leg from Leeds to London Kings Cross which is an LNER train. The Interrail planner says
“Not available from Interrail.eu
This seat reservation can’t be booked on our website. Check 'More information' for other ways to book.”
Firstly, does this mean that I can’t travel by this train at all?
I tried to book a seat with LNER who is the carrier on this train. There doesn’t seem to be a way to book a seat without having a ticket.
Can someone tell me how I book a seat in this case?

