Reservation seat London to Amsterdam not possible

Hello everyone. I have been trying to book the reservation seats for the Eurostar London to Amsterdam. The day is the 2/08 and the time is 8:16am. It's so weird because I do everything and once I want to go to check out, it doesn't allow me to go to the second stage of check out. For a moment I thought the seat were sold out but it doesn't say anything about sold out, in fact it says sold out another time that same day. 

Thanks in advance for your help,