When installing High Sierra on VMWare Fusion, I had to provide the installation files. This is done by finding High Sierra in the App Store, then clicking on “Download”. Once downloaded, the “Install macOS High Sierra.app” is to be dragged from /Applications and into VMWare, as shown here:
Unfortunately, doing so results in the error “Unable to create the installation medium.”, as shown here:
The solution is to drag “Install macOS High Sierra.app” (hold down the left options key before and while you release the app) to a directory owned by your user, such as ~/Downloads, then drag it from there to the “Select the Installation Method” window.