Cubase won't play with low buffer size

its my first post here.

I just bought used mac pro.
i tried to open a cubase session that i was working on with my previous computer but i noticed that as soon as i decreased the buffer size under 1024, the MIDI stopped working. Session was all MIDI.
At the moment i don’t have any audio interface except the built-in that mac pro has.
Näyttökuva 2015-12-20 kello 18.18.07.png