I've had lots of trouble with that website. For one thing, it doesn't make much sense. (Maybe it's just the English version that is confusing.) For another, you have to wait until you are in the month of travel or one prior (in other words, as of now you can book September or October travel but not November travel).
You might look into flying instead - Iberia is good and not much more expensive than train travel; maybe even cheaper. And it saves you a lot of time. AND they have a very good and easy to navigate website. Good luck!.....Jake