Unfortunately iMovie 10 does not have a feature to chapters.
However, iMovie 9 does have the feature allowing you to add chapters. So, if you have iMovie 9 on your computer, you can share out your movie from iMovie 10, import it into iMovie 9, add your chapters there, and share out again.
If you have Garageband version 6.0.5 you can add chapters to your movie there.
Another solution that some contributors to this forum have spoken highly of, is to use an app called MetaDoctor to add chapters. You can purchase it from the Apple app store.
I have read that you can add chapters to the exported Mp4 file with the app Subler, that some have recommended for adding chapters, but have not used it myself. If you want to check it out you can click on their website link here:
https://subler.org/
-- Rich