Which exact trains? (origin, destination, date, departure time per train)

Is there an error message? If so, please post a screenshot. Or else, please post a screenshot of where you get stuck.

A trip like this by train will make you use at LEAST 4 or 5 different trains, will take more as 1 day and some of these are in fact NOT possible to even REServe online.

You have to first check the timings-not yet possible for summer coming, then make the needed REServations (from my memory form the 4 trains: 1 is voluntary, ICE INside DE, 2 is EuroStar to Paris=pay a hefty lot extra, trains 3 and 4; to Barcelona, then Madrid is IMpossible online with pass.

Best site for general info-more aimed at people without passes, is seat61.com-it lists all overland options from UK-so hook on @ convenient point. Seeing your name I guess you are again 1 of the many Svenska in the footsteps of a certain Gretha-alas actual practice @ riding EUR trains makes this often a rather challenging tour.

As in fact a mostly similar topic has been posted here many, many times, more elaborate answers can be found by scrolling down and reading.