You may be attempting to Share (export) the movie from the Event Library. You need to create a Project, drag clips (or segments of clips) from the Event to the Project, then use the Share menu to export the Project. Movie clips can't be exported directly from the Event. Clips you add to a Project reference (point to) the clips in the associated Event (or Events).
For more information, see the following iMovie Help topics:
Start a new video project -
Add video to a project -
The Help topics are also accessible from within iMovie. Click on the iMovie menu item "Help>iMovie Help".