MacBook Pro Catalina 10.15.4 won't update current security update.

MacBook Pro, Catalina 10.15.4, won't download, automatically or manually, the most current security update. The updater (update tool...?) shows an update so I select "now" and "restart." System restarts. The update window is still open and no update has been installed. I'm guessing the update tool is corrupt...? Is this something a novice can diagnose and repair on their own? Thank you.