This one might only get a small stub instead of the full image: https://apps.apple.com/us/app/macos-high-sierra/id1246284741?mt=12

To get the full installer:

• Start the installer, which will download missing stuff
• Quit when prompted to restart
• Copy downloaded files to the right place :

sudo cp -pRv "/macOS Install Data" "/Applications/Install macOS High Sierra.app/Contents/SharedSupport"

Or directly download the missing stuff:

File list: https://swscan.apple.com/content/catalogs/others/index-10.13-10.12-10.11-10.10-10.9-mountainlion-lion-snowleopard-leopard.merged-1.sucatalog.gz

• BaseSystem.dmg
• BaseSystem.chunklist
• InstallInfo.plist
• InstallESDDmg.pkg
• AppleDiagnostics.dmg
• AppleDiagnostics.chunklist

Or :

url=http://swcdn.apple.com/content/downloads/29/03/091-94326/45lbgwa82gbgt7zbgeqlaurw2t9zxl8ku7
curl -R --remote-name-all $url/{BaseSystem,AppleDiagnostics}.{dmg,chunklist} $url/Install{Info.plist,ESDDmg.pkg}

See also

https://www.insanelymac.com/forum/topic/329828-making-a-bootable-high-sierra-usb-installer-entirely-from-scratch-in-windows-or-linux-mint-without-access-to-mac-or-app-store-installerapp/

https://apple.stackexchange.com/questions/299731/macos-high-sierra-media-installer

https://apple.stackexchange.com/questions/324710/install-macos-high-sierra-app-does-not-appear-to-be-a-valid-os-installer-applica

Apple's How to create a bootable installer for macOS :

sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume