Hi,
Try sharing out the project by clicking on the share button (the box with the upward pointing arrow in the top right corner of your screen).
If that doesn't work, try deleting 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 empty library. Reopen your old library to get back to your projects. Now see if you can share out a project. Possibly you may need to reboot to set the changes. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.
You might also try creating a new project and see if you can share from there.
-- Rich