Then relaunch App Store.
If the App Store is not working, you can download the installer directly from Apple. Download macOS Catalina 10.15.7 via Apple Support 5. Troubleshooting: "The Update Cannot Be Installed"
which provides hidden App Store links for older macOS versions like Catalina 10.15. Terminal Command
While Catalina is generally stable, I've encountered a few issues:
Once the download is complete, do not run the installer yet. If you run it, it will upgrade your current OS immediately. To keep the file for future use or clean installs, put it on a USB drive.
If the Mac App Store link fails to load or your current OS prevents the App Store from opening the link, you can use the command-line interface to pull the installer directly from Apple's software distribution servers. Terminal Instructions