At least 14.3 GB of available storage to perform the installation.
You can only download High Sierra from Apple if you previously “purchased” it (free) under your Apple ID, or via official recovery modes on real Macs. Macos High Sierra 10.13.6.iso
Before downloading or installing macOS High Sierra, verify that your target system meets the hardware prerequisites. Minimum 2 GB of RAM (4 GB or more recommended). At least 14
Use Apple's built-in createinstallmedia tool to flash the High Sierra installer files onto the mounted volume. (You will be prompted to enter your Mac's administrator password): Minimum 2 GB of RAM (4 GB or more recommended)
mv ~/Desktop/HighSierra.cdr ~/Desktop/HighSierra.iso
Never download a pre-built ISO from untrusted sources. Use the official Apple methods combined with the Terminal commands detailed above to create your own pristine, malware-free macOS High Sierra 10.13.6.iso in under 20 minutes.