Once you see the seat availability, you can then select the same to book. You may not be able to see the seat availability due to:
You are trying to book the train between <23:30 – 12:30 and 08:00 – 12:00> hours. The Indian Railways reservation system does not allow to book trains during this period through agents.
The seat cannot be booked as all the ‘waitlist’ quota has been filled. In this case, you will see ‘Booking Closed’ in the availability status.