why wont catalina update to the newest update available

Are you trying to update your version of Catalina to 10.15.2.

If you are make sure your mac is connected to your router using a cable rather than relying on WiFi.

Restart your mac, open System Preference> Software Update and try again, do not use the mac while it is downloading.

Alternatively you can download the standalone update from here,

https://support.apple.com/kb/DL2028?viewlocale=en_US&locale=en_US

once downloaded double-click to install.