If you are a course editor, login to the Doctum build environment and go to MANAGE CONTENT > MEDIA to verify that the file in question is located in the media directory. If it is not, you may need to reupload an element and republish your course.
If you are a reviewer, use the comment feature within the preview environment to alert the content editor of any missing media elements.
If you are a learner assigned to a course, contact your local Doctum editor and alert them to the missing media elements.