There are likely engineering works planned that weekend and so tickets have not been put on sale yet. Only the most flexible tickets (valid all day on any train) are currently offered.

The timetable could change a bit, leaving Berlin earlier for example. Check back in a few weeks.

Don't worry this train has optional reservations (like all German trains) and cannot sell out so you're always allowed to board. Seat reservations are recommended on this popular route -> 3€ on ÖBB.