Sometimes, the latest updates to iMovie introduce bugs or remove features that long-time users relied upon. If you recently updated iMovie and found that it crashes on launch or changes your workflow, you might be looking to roll back to the stable version that shipped with macOS Catalina.
If the download continues to fail, you have three alternatives: imovie 10.15.7 download
You are trying to install a newer version onto an old OS, or vice versa. Ensure your macOS is exactly 10.15.7 or 11.x. If you are on High Sierra (10.13) or Mojave (10.14), the last compatible version is iMovie 10.13.x, not 10.15.7. Sometimes, the latest updates to iMovie introduce bugs