Try opening iMovie while holding down the Option and Command keys and selecting to delete preferences in the box that appears. Sometimes that cures export problems. iMovie will open in a new library. Reopen your old library to get back to your projects. Now see if you can export.
Also, sometimes issues are confined to just one project. Try creating a new project and see if you can export from there.
Although you are testing with a small movie, check to confirm that you have sufficient computer disk space for iMovie to function efficiently and sufficient space at the export destination to receive the movie that you are exporting.
Sometimes a corrupt clip can cause export failures. Try using other clips in a project and see if it makes any difference.
-- Rich