Why an AVI video file cannot be imported to iMovie?
I can't tell you why some AVIs can be imported but this one can't, but if you know that the file itself isn't corrupt or anything, I can give you a remedy: Handbrake is an app that can convert almost any video format to MP4, which iMovie should definitely be able to handle. Open Handbrake, select your video file, enter the destination path you want if you don't like the default, and press Start to run the conversion. When it's done, see how iMovie reacts to the MP4. Good luck!