I can't update macOS Catalina 10.15.6 in my laptop

What exact version of macOS are you running at the moment?

If you are running any version of Catalina, try downloading and running the Combo Update:

https://support.apple.com/kb/DL2048?locale=en_US

If it fails, try running it whilst in Safe Mode.

You almost certainly need to clear up some space on your drive.

How much free space is there at the moment?