When some InstructorTasks are stuck (e.g. generating a csv for days in Pending), some tasks can be marked "FAILED" so that new tasks of that type can be created for that course. NOTE: Here be dragons! Support has experience failing .csv reports, email, and grading tasks, but we don't know the impact of handling the full range of instructor tasks. Reach out to Aperture if there are certificate tasks at issue.
Get the task id you're going to fail from the instructor dashboard: courses.edx.org/courses/<course_id>/instructor