The interviewer is trying to learn more about the challenges you currently face, as a leader. If hired, this information will help the company to know where you could use additional training, encouragement, or education.
Every leader has an aspect that is challenging for them. Share your most challenging aspect of leadership but also highlight the steps that you take to alleviate or overcome that challenge.
One common challenge for leaders is the need to terminate an employee. Another problem could be continually motivating unengaged employees. A big challenge could also be sourcing and utilize the best hiring resources to shave down your time spent on reading resumes.