An expired certificate. Apple stopped signing High Sierra in 2021. Fix: Disconnect from the internet. Open Terminal and set the date back to 2019:
Mist is a powerful open-source tool that bypasses App Store restrictions and downloads installers directly from Apple's software update servers. : Download Mist from GitHub Open Mist and go to the Installers macOS High Sierra 10.13.6 Choose to download it as a Disk Image (.dmg) Package (.pkg) Macos High Sierra 10.13.6 Raw Download
, a critical advancement for the burgeoning era of high-definition streaming. III. The Definitive 10.13.6 Version An expired certificate