This is an intentionally tricky question. It's meant to glean insight into whether you can learn from past mistakes. If you can't think of any past errors, it could be an indicator you aren't capable of accepting responsibility for your own mistakes. However, creating a long list of past mishaps could make you look unqualified for the role.
You'll want to answer this question succinctly, and point out an error that doesn't represent a lack of character. Consider one, well-intentioned error you've had in the past, mention it, and then talk about how you grew from that experience.
For instance, you might say, "In my prior role when I first became manager, I took on too many tasks myself and quickly became both overwhelmed, and less efficient in my role. Additionally, my team members were frustrated because they felt there was a lack of collaboration on our team. I quickly recognized I needed to learn to delegate tasks and collaborate on projects with teammates, and became a more successful manager as a result."