Actually been an expert in your field is an additional advantage to your work but I believe it's not mandatory because alot of people or business entrepreneur who are successful today are not expert befor they start but as they do they become one.
Actually it's good and necessary to some extent but it's not mandatory to be because I know no one is 100% perfect and there are certain things that we learn everyday so just try your best and you will learn with time.Yes, it is important and necessary for you to be an expert in the area you work on before you become a manager. The reason is because it is important for a business manager to lead by example. If you don't have enough knowledge of what you're doing there is no way you can be to set examples for the workers to follow.