Seat reservations usually open between 60 and 90 days before but there are some exceptions.

For the Madrid-Lisbon journey please have a look at seat61.com, an excellent website for train travel. It is not straight-forward but still perfectly doable.

Do you have an Interrail/Eurail pass ? This forum is all about them. If you want to book normal tickets (and not a pass), they should be available on the companies' websites again aboit 3 months in advance

Madrid to Lisbon is very badly connected by train. You need three trains and all day to do the journey. (Bear in mind that Madrid to Barcelona is the same distance, and takes 2.5 hours!)

France and Spain are generally not great countries to use a pass. Consider buying individual tickets instead, depending on what else is in your itinerary.

If Madrid isn’t important there is also a routing via Hendaye into Northern Spain and onward. It avoids the expensive routing via Barcelona.