Consider whether you actually need this version, or if you could use a supported macOS version (Ventura, Sonoma, or Sequoia) instead.
sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/install_build macos high sierra 10.13 6 iso download
To download the macOS High Sierra 10.13.6 full installer, the most reliable methods involve using official Apple sources. Apple typically provides these installers as .app files or disk images ( .dmg ), rather than .iso files. Consider whether you actually need this version, or