External Soundcard, play tunes using keyboard/mouse - not possible?

Hi - I am looking to move from Mixxx to DJ Pro on a mac - so I can use spotify. I use a Numark DJIO so I get 2 outputs that go into a physical analogue mixer (where I also plug in some turntables). In Mixxx I can configure output 1/2 and 3/4 to separate channels - use the keyboard to play a track - and the physical analogue mixer to mix.

I am trialling DJ Pro on a mac - I want to do the same thing - I can set up the numark soundcard - no problems - but the play buttons do not work - I think this is because it is expecting an external mixer to drive the sw.

I still want to use DJpro to play tracks - but this is not happening.

Am I right thinking that I can’t use just the external soundcard to get audio out, and the internal mixer to play tracks??