First off, there are a bunch of reasons why a reservation might not be possible. One is that they may block reservations in case of construction or such until the re-routing has been confirmed. Another reason no reservations are possible is that the train is fully booked (and yes, they do oversell).

I'm not a rail travel nerd, but from looking at ICE 27 on your date, it looks as if, in your instance, the train is fully booked in the sense that all 1st class seats have been reserved. So currently, no first class seat reservations are available. Still, I wouldn't be concerned.

The no-show rate for 1st class seat reservations is HUGE. Note that with DB, your train ticket (allowing you to travel on a certain train or route) and the seat reservation are basically completely separate entities. While is does occasionally happen that no 1st class seat is available, it's very rare. Usually, many if not most seat reservations in 1st go unused. Again, it's a very different system to other high-speed train networks where seat reservations and tickets are closely linked.