Your Mac is either too old (pre-2012 models) or too new (M1/M2 chips – Catalina does not support Apple Silicon).

This specific update (released September 24, 2020) focused on critical system stability and bug fixes: Download macOS Catalina 10.15.7 Combo Update

The safest, fastest way to get the is Apple’s softwareupdate Terminal command. It gives you the genuine, unmodified installer directly from Apple’s content delivery network.