virtualbox.org

Host: HighSierra 10.13.6
Guest: HighSierra 10.13.6

I load the ISO file to the virtual optical drive. When VM starts always shows a "Shell>" prompt, no matter if EFI is enabled or disabled.

Any idea?

Although you are using a macOS host, you are trying to install a macOS Guest, so your question would be better suited for the Mac OS X Guests forum.

In that forum there is a topic [HowTo] Install OSX 10.13 in a VM

If you read through that topic I think you will find your questions are answered, but if you do require further help, please attch the VBox.log file for your session to your post.

Switched to Mac Guests.