Cannot buy units in tutorial 3

[OldFocker]

In the assault tutorial to capture encircled hexes the 'Purchase' button is always ghosted out so am unable to buy units and deploy them.

[Yogi the Great]

One of the tutorials (may be 3) does not allow purchase and place. You have to capture the required hexes and then other units will automatically appear. You aren't able to get to all of the required hexes but just go after the ones you can and new units will later appear for the others.