You don't need technical knowledge, you need skills. Both my husband and I have gone into management jobs and done well in them with minimal knowledge of what the department did in terms of engineering/nursing/physiotherapy/oil refining.
You do need to be able to pick the basics up quickly but there will be professionals around who can brief, coach and advise. I used to manage clinicians who were much better clinicians than I ever was.
Managers and heads of departments need management skills like change, planning, person management vision....you know the kind of thing and these will depend on what the department needs to do and what it is like in terms of efficiency, safety, standards, delivering its objectives and so on.