Stuck with old version of iMovie (please help!)

I am assuming that you deleted iMovie 10.

To redownload your deleted iMovie 10, sign in to the app store and open your

purchases folder by clicking on your name at the bottom of the app store side

bar. You will see your purchases displayed, including iMovie 10, if you had it

before on your Mac. Redownload/update iMovie from there. If unsuccessful,

try using the app store search box to search for "iMovie 10 app" and see if you

can download it from among the other apps listed.

Also, if you keep a Time Machine backup you could restore your iMovie 10 app

at a point in time preceding when you deleted it.

-- Rich

What you can do now is upgrade to Mojave. You 2014 Mac Air qualifies. Here's a link that explains how to upgrade to Mojave.

https://support.apple.com/en-us/HT210190

Once you upgrade to Mojave you can then upgrade to current version iMovie 10. That probably will cure your CompressorKit.Error.

-- Rich

If you carry out the instructions from Rich regarding downloading iMovie from your Purchased page you will get a message similar to this.

This message is not referring to your older version 9.0.9 but an older version of iMovie 10 that is older than the latest release.

The download will provide you with the latest version of iMovie 10 that is compatible with your current OS.

This could be a later version than you had before if you were not running the latest version for your OS at that time, although this should not be an issue.