Slitherine

I am in the Bardia scenario and it shows that I have one core slot available. It allows me to purchase a unit for the British, but not under Australia. When I switch to the Australian flag, it then tells me "No slots available" and the purchase button is grayed out. Are you not allowed to buy units under minor allied nations? I can't accept this as true so what gives?

You are only allowed to have British and later on American units in your core. The other commonwealth nations like f.e. Australia, New Zeleand, India are only used as auxiliary troops.