(Constructor code grabbed from OP comment.)
In your constructor, it appears that you just initializing a local variable within that method, not the field _list.
public Club(string clubName, Coach mycoach)
{
List<Player> _list = new List<Player>();
ClubName = clubName;
Mycoach = mycoach;
}
Try changing
List<Player> _list = new List<Player>();
to:
_list = new List<Player>();