Installer for MacOS High Sierra.

I discovered why I could not download any version of macOS, such as macOS High Sierra or macOS Catalina. Starting with systems running High Sierra or Catalina, the attempt to get a macOS installer is treated as an update, hence if one is running Catalina then one cannot get earlier version.. The only way to download previous versions or the current version installer is to use an apple system running Sierra or an earlier version. Fortunately I have an older machine running an ancient MacOS. I will tryout my theory and let you know.

Unfortunately when I get to the App store and toggle "get", I receive the message "not found".

I need some other link, this one does not work. I get the message, "not found" when I toggle the the "get" button.

A little clarification. I mistakenly stated that I was running Catalina. I am running Mojave.

However my same conclusion holds, at least until I try it in a couple of weeks.

What do you mean “if true”. A little more information please.