Can't download macOS High Sierra once again
You can use the link available on the Apple Support document, How to upgrade to macOS High Sierra to download the installer for macOS High Sierra 10.13.6.


  4. Download macOS High Sierra
  
  
    For the strongest security and latest features, find out whether you can upgrade to macOS Mojave, the latest version of the Mac operating system.
  
  
  If you still need macOS High Sierra, use this App Store link: Get macOS High Sierra.


The installer is downloaded in /Applications directory and is typically around 6GB in size. If needed, delete the installer currently present in /Applications directory.