If you have never downloaded iMovie before, the App Store may simply tell you it's incompatible.
If you see "Open," it means the App Store detects that iMovie is already installed. imovie 10.2.5 download
If you’re on , the last compatible iMovie is 10.1.14 (search App Store → it will offer “last compatible version”). If you have never downloaded iMovie before, the
If you see a message “This app requires macOS 10.15 or later,” then your Mac cannot download 10.2.5 – you’d need an even older version (see Method 3). imovie 10.2.5 download