Damaged macOS Mojave installer
As per this discussion on Apple support community, you should be able to overcome the issue by deleting InstallInfo.plist file from within the macOS Mojave installer app.


  
  Go to your applications folder.
  Find the installer for macOS Mojave.
  Right click on the installer and click "Show Package Contents".
  Click on folder named "Contents".
  Click on the folder named "SharedSupport".
  Delete the file named "InstallInfo.plist".
  Enter your administer password to confirm.
  Now open the installer.