Your Mac model does not officially support High Sierra, or the installer is corrupted.
No. Apple does not provide ISO images. You must download the .app installer and convert it if needed. download macos high sierra 1012 dmg exclusive
Using a macOS High Sierra 10.13 DMG is the best way to breathe new life into an aging Mac. By utilizing APFS and Metal 2, your hardware will feel faster and more responsive than it did on older versions of OS X. Always ensure you back up your data via Time Machine before performing any major OS installation. If you'd like to move forward, let me know: What are you using? Your Mac model does not officially support High
In Terminal, run: