Take a look at this thread and see if the solution there helps you:
https://discussions.apple.com/thread/7073179
The solution in the above thread involves creating a new iMovie library and copying your project over to it, then share out from the new library.
Other things you can try:
Delete preferences. To delete preferences, open iMovie while holding down the Option and Command keys and selecting 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. Now see if you can share out.
Another thing you can try is to boot up in Safe Mode (see Finder help menu for instructions) and, after everything loads, immediately shut down and reboot in normal mode. Sometimes that clears up conflicts.
-- Rich