Here is the most reliable guide on how to safely source and download the installer. Method 1: The Mac App Store (The Official Way)
Wait for the download to finish. The file will be placed in /Applications . Method 3: Safari Download (For older macOS versions) download macos catalina 10.15.7 full installer
date 0101010119
Disconnect your Mac from the internet, open Terminal while in Recovery mode or installer boot, and change the system date to a time closer to the release of Catalina (e.g., type date 1025102020 for October 25, 2020) and try running the installer again. "The requested version of macOS is not available" Here is the most reliable guide on how
and click Continue.
If you are on a newer Mac, you can force the download using Terminal. Open (Applications > Utilities). type date 1025102020 for October 25