If your Mac is stuck with High Sierra , you can’t upgrade it and you want to install iMovie ? follow the steps below:

Installing iMovie9

# 1. Download the iMovie 9.0.9 update package
curl 'https://updates.cdn-apple.com/2019/cert/041-98154-20191017-1fbef983-8d87-49af-83b1-edecc007ce2e/iMovie9.0.9Update.dmg' -o ~/Downloads/iMovie9.0.9Update.dmg

# 2. Mount the dmg
open ~/Downloads/iMovie9.0.9Update.dmg

# 3. Expand (unarchive) the pkg
pkgutil --expand /Volumes/iMovie\ 9.0.9/iMovie9.0.9Update.pkg ~/Downloads/iMovie/

# 4. Rename Payload to Payload.zip
mv ~/Downloads/iMovie/iMovie9.0.9Update.pkg/Payload ~/Downloads/iMovie/iMovie9.0.9Update.pkg/Payload.zip

# 5. Unpack Payload.zip (it will get named Payload 2)
open ~/Downloads/iMovie/iMovie9.0.9Update.pkg/Payload.zip

# 6. Move iMovie.app to the Applications folder
mv ~/Downloads/iMovie/iMovie9.0.9Update.pkg/Payload\ 2/Applications/iMovie.app /Applications/iMovie\ 9.0.9.app

# Enjoy!
open /Applications/iMovie\ 9.0.9.app

If you want to upgrade to iMovie 10, follow the steps below: