Hello,
I've just booked train tickets for Munich to Vienna on DB. I booked first class, but there were no seat reservations available. Does this mean there is a chance I will have to stand on the train? Or does it just mean that I'll have a first class seat but I won't know where to sit ahead of time?
It's free for first class passengers.
That's true only if you book your seat together with a flex price ticket.
You can still can try to book seats: On the DB website search for your booked connection and click on "book seat only".
Unfortunately, you will have to pay extra for a separate seat reservation, even if you already have a Flex Price ticket