I found the answer, thanks to a comment by @phoog.
The German version of the website says “Der Zug ist vorübergehend nicht reservierbar.”, which seems to translate to “The train is temporarily not reservable.”.
This suggested that a ticket can be booked, but a seat cannot not be reserved. I tried by going to the end of the process (I realized there was one more step before payment that I had missed), and indeed the English version also said that reserving seats is temporarily unavailable.
Credit:stackoverflow.com‘
4 Mar, 2024
5 Mar, 2024