macOS may have quarantined SketchUp. If you’re sure that you copied the SketchUp 2021 folder to Applications, and you ejected the disk image before running SketchUp from the folder in Applications, try these steps:
- Open Terminal (it’s in Applications/Utilities).
- Copy this text, and paste it into the Terminal window, and press Return:
xattr -d com.apple.quarantine /Applications/SketchUp\ 2021/SketchUp.app
That should take SketchUp out of quarantine, and hopefully it will open ok.