How do you create a culture where employees do not fear to speak up?

As a leader, you have to be honest with yourself. Most often, if you have created an argument for the sake of argument, you already know it, and it is on you as a leader to avoid it. People do not like to be criticized. Set up your company in a way that people around you are able to give you honest feedback, and you can share honestly with them; measure it. Ask your employees if their best ideas got heard, if they felt safe sharing those ideas, what happened when they were discussing those ideas...etc.