Softwareupdate --fetch-full-installer

@AJPinto Have you tried https://github.com/munki/macadmin-scripts/blob/main/installinstallmacos.py to see if it can grab a Catalina installer? The oldest version of Catalina it's showing for me is 10.15.3 though. Since it finds Mojave and High Sierra installers that implies that 10.15.1 may no longer be in Apple's Software Update catalog.

I would say the first thing to verify is your syntax, because to download Catalina one would use:

softwareupdate --fetch-full-installer --full-installer-version 10.15.4

with 10.15.4 being the oldest version officially listed for download as of now with the appropriate command:

softwareupdate --list-full-installers

and downloads just fine — just verified. With incorrect syntax that „version” parameter gets ignored and the binary falls back to the latest downloadable version, which is indeed the 11.6 Big Sur.

@AJPinto are you trying to download from an Apple Silicon device? If so you can't download anything before Big Sur because that is the first operating system to support Silicon. Try it from an Intel device if you have one handy.

@bartlomiejsojka @AJPinto Bart is correct, the first command he is listing is the right way to grab a specific installer.  The regular --fetch-full-install will try to grab whatever the newest release is, which I have found can be a bit hit or miss.  I like to specify the exact version no matter what.