You cannot make a bootable High Sierra flash drive installer unless you have downloaded the full sized Install macOS High Sierra installer, and it is located in your Applications folder. The installer should be about 5GBs.
Bootable USB Installers for OS X Mavericks, Yosemite, El Capitan, and Sierra
First, review this introductory article: Create a bootable installer for macOS. Second, see this How To outline for creating a bootable El Capitan installer. Simply replace the Terminal command with the one from the preceding article by copying it into the Terminal window. You will need an 8GB or larger USB flash drive that has been partitioned and formatted for use with OS X.
Drive Partition and Format
- Open Disk Utility in the Utilities' folder.
- After Disk Utility loads select the drive (out-dented entry with the mfg.'s ID and size) from the side list.
- Click on the Erase tab in the Disk Utility toolbar. Name the drive, "MyVolume". <---- IMPORTANT!
- In the drop down panel set the partition scheme to GUID. Set the Format type to Mac OS Extended (Journaled.)
- Click on the Apply button and wait for the Done button to activate. When it does click on it.
- Quit Disk Utility.
Open the Terminal in the Utilities' folder. Choose the appropriate command line (in red) depending upon what OS X installer you want. Paste that entire command line from below at the Terminal's prompt:
Command for macOS High Sierra:
sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ macOS\ High\ Sierra.app
Command for macOS Sierra:
sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ macOS\ Sierra.app
Command for El Capitan:
sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app
Command for Yosemite:
sudo /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ Yosemite.app
Command for Mavericks:
sudo /Applications/Install\ OS\ X\ Mavericks.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ Mavericks.app
Press RETURN. You will be asked for your admin password. It will not echo to the Terminal window. Then press RETURN again. Wait for the return of the Terminal prompt signifying the process has completed. It takes quite some time to finish. Be patient.