As The Muse states, a manager should know about the latest trends and most up-to-date developments on the technical level of the area in which their team works. Remember to keep developing your hard skills so you can continue helping your employees.
There is no such thing as too much of communication skills. Leading people is mostly about interpersonal communication, which is why you should never stop improving your speech, emotional intelligence and methods of effective communication with team members.
Knowledge of the legal aspects of the relationship between employer and employee is not just for the legal and HR departments. As a manager, you will use this knowledge not only in extreme situations, such as when an employee is resigning or failing to fulfil their duties, but also in your everyday interaction with team members.
Time management and organisation of work
Time management and organisation of work are areas that can always use improvement. Even small changes and development in these areas may help you significantly improve your productivity, both on the individual and team level.