Because it is fully booked would be the most logical answer.
Night trains are busy and often booked out in the summer.
Those night trains don’t have huge capacity, 1 or 2 seated carriages, 2 or 3 couchette carriages and 1 sleeper car.
But I could add it to my trip at first?
But I could add it to my trip at first?
You can add any train to “My Trip” at any time.
Compulsory reservstion trains also require reservations and this one is full.
There is an ALT ICE HH-MÜnchen, ICE with seats only, it may even bypass this NJ somewhere-but then you need, if applicable, a new passday for next leg M->INns. This ICE is easily bookable as seatonly on bahn.com-if this site works again.
Dammtor is a strange place to board it-unless you maybe have a HTL nearby/ Common is Altona=start point or HBF=main and overcrowded