macOS Mojave: createinstallmedia: command not found
It appears that you do not have the installer for macOS Mojave downloaded on your system. Follow the steps listed in the section Download macOS in the linked Support document, How to create a bootable installer for macOS, to download the installer locally on your Mac before running the command:


  Download macOS
  
  
  Download a macOS installer, such as macOS Mojave or macOS High Sierra.
  
  To download macOS Mojave or High Sierra for this purpose, download from a Mac that is using macOS Sierra 10.12.5 or later, or El Capitan 10.11.6. Enterprise administrators, please download from Apple, not a locally hosted software-update server.
  When the macOS installer opens, quit it without continuing installation.
  Find the installer in your Applications folder as a single ”Install” file, such as Install macOS Mojave.
  


The installer is typically around 6GB in size. You can verify it by going to /Applications directory, selecting the app bundle and using Command + I keyboard shortcut to view info.



After you have the installer available locally, you can connect an external USB drive, run the mentioned command and create a bootable installer.



Note: The macOS installer can be placed anywhere on disk. Adjust the path of the createinstallmedia command accordingly to create the bootable media.