I can't say why that is. However, if this is on the Eurail website, then please check this thread for better places to book reservations:

If you have question after looking at this thread, then please specify route, date and departure time for each train you'd like to book.

There have been a very few same-same postings about this lately.

BUT I guess you are Chinese? In that case-very often, also due to the bizarre ways that China itself handles foreign things (probably still afraid of foreign devils) and the thing named weChat unknown outside of that country often causes trouble. Of course the Chines themselves see all this quite the other way.

many touristy sights that receive many mainland Chinese need to have installed special other ways to pay with cards-at great expense.

To make RES for trains-always better to use other ways as via app-cheaper too. Best choice is mostly via the railway of that country-but this may vary in how easy that is.

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