macOS High Sierra 10.13.6 was a significant milestone for several reasons:
If you must use High Sierra (e.g., for 32-bit apps or legacy hardware): macos high sierra 10136 dmg
Cause: The certificate expired (Apple’s old installer certificates were set to expire in 2019–2020). macOS High Sierra 10
Fix: Disconnect from the internet, set the date back to 2018 in Terminal, then run the installer. After installation, re-enable automatic date/time
date 0101010118 # Sets date to Jan 1 2018
After installation, re-enable automatic date/time.
