They aren't sure what the job is. They don't know how to do the job. A person or situation interferes with employees' ability to do the job.
Perfectionists set high standards and push themselves hard.
That's fine for self-motivation and career moves, but it can cause problems if they supervise others. Help them see and acknowledge that all people make mistakes, and that occasional mishaps don't preclude success. Coach them to recognize the difference between who they are and what they do for a living. Encourage them to publicly recognize what goes well and not dwell on every error. Remind them that steady praise will improve the morale of those they supervise.