The macOS High Sierra installer downloaded from the Mac App Store (look for "Install macOS High Sierra.app" in your Applications folder). At least 15 GB of free storage space. Step-by-Step Terminal Instructions
What (Windows, Mac, or Linux) are you currently using to prepare the file? Share public link macos high sierra 10.13.5.iso
This is the most common error when installing older macOS versions today. It occurs because Apple's security certificates embedded within the old installer have expired. The macOS High Sierra installer downloaded from the