Yeah, that's great advice for anyone who has the ability to download the full installer without encountering the error described here. I don't really understand why you are so insistent on mentioning it here though. Can you appreciate how your recommendation might be frustrating to hear? Do you often visit threads where people are experiencing problems just to inform the world that it's not a problem you've ever faced?
First, why are you getting into a "pissing" contest with me? Secondly, I am not narrow minded, and I do not visit threads just to "bash" folks, not criticize them. I just make constructive, helpful statements. Statements like you are making above are definitely not informative, nor helpful.
Do you recognize that the Software Update Preference pane is the way a person downloads the full installer, and that the App Store technique you recommended does not work the way you think it does in Catalina? You cannot download the installer from the App Store any more, clicking "Get" just redirects the user to System Preferences.
That is not true! Read this:
"How to upgrade to Macos Catalina?
You can download and install
macOS Catalina from the App Store on your Mac. Open up the App Store in your current version of
macOS, then search for
macOS Catalina. Click the button to install, and when a window appears, click "Continue" to begin the process."
Via the Software Update Preference Panel, one is downloading an update of the new Mac OS to the prior one. So, when that is downloaded, the installation process begins immediately, and updates the prior "eligible" Mac OS to the new one. If one instead downloads the FULL Installer, that is NOT done through that Preference pane. One does that through the App Store. For the subsequent installation, it is NOT required to have a prior Mac OS on the machine.
I always download the full installer, stop the subsequent installation process, make a copy of that installer file in another location on my machine, and then remove the one that landed in the Applications folder. After doing one final cleanup of my current system (and that new Installation file will be there), I make a SuperDuper! backup. Next, I boot from that SuperDuper! backup, use Disk Utility there to Erase and Format the internal SSD on the respective Mac, navigate to that (copied) Mac OS full installation file, and launch it. That does a full, clean, "virgin" installation of that Mac OS onto the internal SSD. Finally, I am offered the opportunity to migrate/copy needed files, folders, settings, etc. from that just completed backup, and away it goes. When I restart the Mac, I have that new Mac OS, along with all my apps, settings, etc. (I usually have 2 more tasks to do after that completes: install the new, Mac OS specific version of Onyx, and make the TechTool Pro eDrive).
I have actually done all that "kind of" in reverse, where I have done the installation of Catalina on an external SSD, then migrate/copy needed information. When I boot my Mac from that external SSD, I can use it to test out Catalina. That is what I have recently done.
Right now, the full OS 10.1.5.1 installation file is "not available" from the App Store. But no sweat, as I don't need it immediately, especially since I am still testing Catalina on that external SSD (two of my critical apps do not yet have Catalina updates available: SuperDuper!, and TechTool Pro).