the undo function on iMovie not working

What edit are you trying to undo?

The Edit/Undo feature will not undo an edit after you have closed iMovie and then reopened it. Also, if you have done intervening edits, you will need to undo the most recent edit and keep working backwards to undo each previous edit.

If Edit/Undo is not working then possibly you need only to delete/reset preferences. To delete/reset 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 library. Reopen your old library to get back to your projects. 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 can also try placing your media into a newly created project and see if the Edit/Undo feature works there.

If still it won't work, you may need to replace the editied clip with your original clip.

-- Rich

Hello ~ Is there a technical question we can help you with?

~Katana-San~

I hope it solves it!

-- Rich