Create a bootable installer for High Sierra under macOS Catalina Question
I'm trying to create a bootable installer for High Sierra 10.13.6 under Catalina. To create the installer app I invoked the softwareupdate command this way:  

sudo softwareupdate --fetch-full-installer --full-installer-version 10.13.6  

Obviously the download started. Then after reporting a progress of about 60% it failed:

Downloading and installing 10.13.6 installer
Install failed with error: Update not found 


Any idea what went wrong here? Any advice is highly welcome.

Hardware-Software-Environment  

MacBook Pro 2019 running macOS 10.15.4 Catalina. There is no encryption enabled.

Background
As discussed in the first two posts mentioned below you may start with the recipe from this article How to upgrade to macOS High Sierra. This provides you with theApp Store link. At least for me, when initiating the download from a MacBook Pro running OS X 10.11.6 El Capitan, this resulted in creating only the stub installer (as described, among others, in the references below). 

References
These two posts, although somewhat more general, are closely related to my above question:

How do I download macOS High Sierra for offline install?  

macOS High Sierra media installer 

Discussing use cases of the option --fetch-full-installer of the  softwareupdatecommand new in macOS Catalina
https://derflounder.wordpress.com/2019/10/28/downloading-macos-installers-with-updated-signing-certificates-on-macos-catalina/
You are right - this seems to work only if the executing mac does support running the target OS. A download of 10.13.6 succeeds on MacMini7,1 and in a VM but fails on a MacMini8,1. Given that, it is likely you also get hardware specific builds if these are available for a given OS.

The installinstallmacos.py script lists all available options and does show Unsupported Board ID or Unsupported macOS version in cases that are not supported but it is able to download the installers from supposably the same source as the softwareupdate tool.

So either run softwareupdate --fetch-full-installer on compatible hardware, in a VM or use installinstallmacos.py.