Ask HN: What's the one thing a new engineering manager must learn or know?

Your responsibility is not to tell people what to do. Your responsibility is to help them communicate with the rest of the company (in both directions); to coach them to become better engineers (including developing their soft skills); and to guide them to make better decisions based on the information and big-picture priorities you share with them.

Theory of constraints.

https://en.wikipedia.org/wiki/Theory_of_constraints

That the first thing to build is trust. The disease that plagues companies is not communicating out of fear you'll get in trouble if something is broken or didn't get done. Build trust to counteract fear.