Need help - security update won't work

Hi guys,
My computer works great and running macOS 10.15.7 (19H15) flawlessly.
I get a message about Security Update 2021-005. When I click "update", it is downloaded, the computer boots but just goes back to the login screen, Nothing happens.

So I read in the forum here and downloaded the update from support.apple.com/downloads, but the same thing happens.

Restart and back to the login screen.

The version remains macOS 10.15.7 (19H15).

What am I doing wrong? Thank you for your help and...

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Author: danroberto