Have you checked that the carrige number and seat nmbers are available for that train? Normally you don't need to enter the seat numbers yourself. 

It didn't give me any option to check unfortunately , it only said input your seats and carriage!

No matter what numbers I put in on this website (M.cd.cz) it keeps saying this error. Any ideas what I need to put to pay for a seat reservation? It is a direct train

[1]

Railjet 257 is running from July. Reservations are just possible at the moment until 5.6. So I think you try to early (even there's no date shown in your picture).

[2]

There's no carriage 2 on this train, just 21 to 27. 21 to 25 2nd class, 26 / 27 1st class.

>3]

It's also possible to book without concrete numbers; but because of 1] I don't assume that's the problem. 

It always helps to mention the travel date...