Which day? and where are you trying it? Interrail /Eurail or directly with the railcompanies?

Here a article with a manual how to reserve directly with DB (4€ per journey in 2nd class) or OBB (3€ per train) :)
 

Thanks for the quick feedback. I’ve made reservations for 4 of our 5 Global Pass days and did not have any problems making reservations for the other destinations. There are 8 of us and we need to travel from Munich to Vienna on Thursday Sept. 2. I tried all four available trains listed but each one stops me and says choose another train? Would like to book a Railjet that takes about 4 hours. Thanks

Hey!

Just tried on oebb.at and could book without problems a reservation for 2nd September. There are still lots of empty seats, so should not be a problem.

On which website did you try it?

Martin