When I start up a new game it doesn't give me a choice between to two. I mean, I see the "Head Coach" box there, it just wont allow me to select it. Is there a reason for this?

You don't need to select Head Coach if you are playing in global mode - you have control over everything GM and HC. If you play in local mode then you would only control your job function and then there is the need to decide if you want HC duties as well or not.