iMovie isn't functioning correctly. Often deleting preferences will resolve the issue
To delete preferences, open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.
In case that the problem is confined to just this one project, try creating a new project and see if you can share out from there.
-- Rich