Cant finish tutorial

[ICanBreakdance]

so, I just bought this game and I have no idea how to play it so I ran the tutorial...everything goes fine until i get to the stage where I have to build a farm. I move the worker to the correct square and that Sid fella tells me to click "Build Farm" in the "Unit menu". And I would love nothing more than to do that but the square is grayed out and it is un clickable. Whats going on?