You can't book reservations without a ticket on the Trainline website.

Domestic reservations for Spain can't be booked online at all. RENFE stopped that. Fot he remaining possibilities, see:

Reservations for Italian day trains can be made on tickets.oebb.at. Do NOT use "Seat reservation only” but add the Interrail/Eurail discount and then select "One-way tickets and day tickets”.

Thanks very much that's a great help,Regards Dave

Thanks for your answers guys, we have booked tickets going through France Italy and Spain using the app but I can still not get tickets from London Paddington to London st Pancras using the app  for some reason it keeps coming up with no routes available?

Either you can take the non Included direct Subway :) 

Or you use the Elizabeth Line (included) till Farrington and use from there the “Thameslink” to St Pancras The Eurail App doesnt show these trains you have to add them manually :) 

It looks like the Elizabeth Line is not in the app but Thameslink is.

Thanks Seewulf that clears it up I will add it manually,cheers Davejacky.

Cheers Rvdborgt 

Really? Couldnt find any train from Luton not Luton Parkway] (my Hotel)  to London :D