Attempting to share a video in iMovie '13 results in error -50
That error isn't the most helpful, but you can do several things to rule out some easy problems:


Export onto a different drive - plug in an external with at least 10 GB of space assuming your movie isn't more than 20 minutes long.
Change the size and try re-exporting to your disk.
Open the console app and search All Messages for iMovie errors and post them if they are not clear to you on next steps.