If you are already on macOS Catalina (10.15) or later, you can force the download of the latest full installer version via the command line. : Go to Applications > Utilities > Terminal .
Before proceeding, check the file you downloaded: download macos catalina 10.15.7 full installer
Your Mac will ask to open the App Store. Click Open . Download: Click the Get or Download button. If you are already on macOS Catalina (10
Replace [External Drive Name] with the actual name of your external drive. Click Open
softwareupdate --fetch-full-installer --full-installer-version 10.15.7
The System Settings (or System Preferences) window will open, asking if you want to download macOS Catalina. Confirm the download.
If the App Store isn't cooperating, you can force the download using a simple Terminal command. This pulls the full Install macOS Catalina.app (approx. 8GB) directly from Apple's servers: (Command + Space, type "Terminal"). Copy and paste the following command: