imovie for 10.11.6

Unfortunately you can only download an older compatible version if you have already downloaded a version of it before.

If you have already downloaded it, then it will appear in your Purchased list.

If you have a friend with a Mac running 10.13.6 or later you may find a solution here.

Not sure if this works as OP didn't reply.

https://discussions.apple.com/thread/250046371

If you are able to try this method then it would be good to know the outcome.

Get it on the Mac App Store here: iMovie for macOS

It should ask you if you want the latest available version available for your operating system.