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.

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.