If the App Store method is unavailable or fails, you can force the download using a Terminal command (available on macOS Catalina 10.15 or later). Where is the link for Catalina 10.15?! - Apple Community
If you already have Catalina and only need the 10.15.7 patch, you can download the macOS Catalina 10.15.7 Combo Update directly as a DMG. Step 2: Create a DMG from the Installer download macos catalina 10157 dmg file better