The ticket is not bookable - why not?
If you see a connection, but there is no green button to pay for it, then the connection is not available. This could have different reasons: a fully booked bus, the operator canceled the trip, the operator changed the schedule, you are trying to book too early or too late - you name it. Or you selected a route for a train that can not be booked online, e.g. the connection from Bangkok to Ayuttaya.
If it is not bookable there is nothing you can do about it. Please pick a different connection.