The High Sierra OS Kernel cannot find access to the USB disk to take control of further booting. The denial of access may be due to the USB port to which you have inserted the HighSierra Installer, may be USB 3.0 and not working yet. There is no data transfer from the USB Flash Drive to the RAM.
What to do?
Try using a different USB port, preferably at the rear, and if your USB disk is USB 2.0 use a USB 2.0 port at the back and try again. Very Occasionally the fault may be with the USB disk itself. You could also remake the installer on a new USB 2.0 drive.
Note: If you are installing HS to a laptop you should be posting in the Laptop forum to get help, not in desktop support.