Installing High Sierra on an external SSD using MacBook Pro 16 running Catalina

Hello All,
I’m trying to help a friend by re-installing High Sierra on a new SSD for his 2010 iMac (the HD failed so I can’t clone it). However, with my Mac not being able to run High Sierra I’m unable to download it and install it to the external SSD. I did locate a version of Sierra thinking I could just get it booted up and then upgrade, but when I try to create a bootable USB, I get the error using Install Disk Creator

Invalid macOS Installer
The selected item does not appear to be a valid macOS Installer.

It is lacking the file “Contents/SharedSupport/InstallESD.dmg”

Does anyone have any suggestions to overcome these issues?

There’s also