If you have deleted preferences, reinstalled iMovie, and converted the clip as suggested -- and still no luck -- then possibly a visit to the genius bar at your local Apple store would be in order. They have testing equipment and technical expertise that is not available on a support community website like this one.
There can be many reasons for share failures in addition to the ones so far mentioned in this thread. These would include insufficient space in the destination drive, corruption in the clip, incompatible frame rate (50fps), incompatible resolution (2.7k vids), use of the lpcm audio codec in Mp4, and so on. Possibly some conflict on your computer would cause it as well. I and many others are running iMovie 10.1.9 with High Sierra and are not having this problem, indicating that the issue probably is not indigenous to the software. That's not to question in any way those who do have the export issue.
The only other diagnostic things that I can suggest, other than the genius bar, are the following:
1. Open iMovie in a new user account and see if the problem persists there.
2. Open iMovie in a newly created library.
3. Boot up in Safe Mode (see Help Menu) and, after everything loads, immediately shut down and reboot in normal mode. That might clear out any conflicts that exist.
4. Try sharing to Theater.
5. Share out in Best Quality (pro res).
6. Confirm that your external drive is formatted Mac OS Extended (Journaled).
7. Reinstall your operating system (back up everything first).
I hope that you and any others who might be experiencing this problem can find a solution. Perhaps another contributor to this forum who has resolved it will chime in here. Meanwhile, best of luck with this.
-- Rich