On older hardware (2010–2015 Macs), High Sierra 10.13.6 runs faster than Mojave or Catalina. It uses less RAM and puts less strain on the GPU.
High Sierra automatically converts SSDs to APFS. To check: macos high sierra 10136 dmg install
This is typically a language or localization file issue. Integrity checks
This is the most common error when trying to open the DMG or run the installer from an old download. Check code signatures on the extracted Installer app:
You need a DMG installer of 10.13.6 if your Mac falls into the following range:
Note: If your Mac is from 2011 or earlier, High Sierra is likely the final officially supported OS.