Which trains are you talking about exactly? Always trust the website of the company itself (LNER), not the Eurail/interrail planner or app.

You can board with reservation and + global pass. Did this in summer, no problem at all. Best reserve at LNER https://www.lner.co.uk/travel-information/make-a-reservation/ or GWR https://www.gwr.com/your-tickets/seat-reservations

In the UK you can almost board every train with your pass. Excellent Interrail/Eurail country.

You can use LNER trains without reservations. They have a specific coach that is unreserved. They do however send incorrect timetable data to the rest of the world indicating that reservations are mandatory instead of recommended.

Reservations are however free of charge. Ignore ACP Rail (I don't know why Eurail are pushing that website), but use GWR (using these steps) or LNER's own website to book reservations free of charge.

@BrendanDB I am talking about train from London King Cross to Edinburgh on 29th December. The train runs at 6:15 and reaches Edinburgh at 11:14. But eurail website is not showing this train (ONLY LNER WEBSITE IS SHOWING) thats why I asked that can I travel on it with my Eurail global pass and a seat reservation. And how can I add that trip on my pass if it isn’t visible in the app?

You can add journeys manually too. Make sure you add in all the details you find on the LNER website. Departure time, train number,...

What the Eurail website or the rail planner app is showing is not relevant. They're not reliable because they don't get enough updates.

For great Britain, check timetables on nationalrail.co.uk (or the operator's own website).

In GB they keep the train number a secret… but fortunately, it's not mandatory when adding a train manually.

Always doing things slightly different those Brits.

Anyway, this is a post on adding manual trips in case of mistakes or missing trains in the rail planner app:

@rvdborgt @BrendanDB Thankyou guys, I have added the trip manually and have made free seat reservations from LNER website. hope everything goes as planned.

LNER, as most other mainline UK operators, is very flexible for Interrailers, and allows reservations up to a few minutes before departure. I would always recommend making a reservation for LNER because the one coach without reservations is only one of nine and very rapidly fills. The rest of the train will have seats available but usually singles or occupied from intermediate stations so you get ejected.

The time you are travelling the trains to Scotland will be very busy with many travelling for Hogmanay, so you were wise to make that reservation.