Reservations in Great Britain are always free and optional, regardless of what Rail Planner says (the railway companies send incorrect data to the database).

You can get Grand Central reservations for free on the GWR and Transpennine Express websites or at any ticket counter, like I suggested in your previous post asking the same question: 

Logging to the train on Rail Planner is an entirely separate step. You can log any train whether you have a seat reservation or not. When mandatory it will get checked onboard.

Have a read of these pages:

https://www.seat61.com/how-to-use-an-interrail-pass.htm

https://www.seat61.com/interrail-and-eurail-reservations.htm

All seat reservations are optional for day trains in GB but some operators choose to send incorrect information. Grand Central seems to be one those operators.

If you would like a reservation though, you can book them free of charge at any ticket office or via GWR:

https://www.seat61.com/interrail-and-eurail-reservations.htm#great-britain