10.15.7 is the final version of Catalina. It includes all security patches and bug fixes released before Apple moved on to Big Sur. If you are going to run Catalina, use an older version like 10.15.0 or 10.15.3. Stick with 10.15.7.
A prompt will ask if you want to download macOS Catalina. Click Download . download catalina 10.15.7 full installer
If the App Store method fails, you can force your Mac to fetch the installer using the Command Line. This is often the most successful method for users on newer hardware. Open (Command + Space, type "Terminal"). download catalina 10.15.7 full installer