Hello Thiequuepie25
IC3 is the line number; all trains serving the same route are IC3 trains.
On the other hand, 574 is the train / course number, it is unique. In fact, the seat reservation would be for the train 574 that is part of the IC3 line.
I do not know what you did wrong; I am able to reserve seats, although there is no need to reserve seats for a Wednesday afternoon; no local would ever reserve a seat. Save that money and get you a coffee in the dining car instead.

Kind regards,
Roland