Can’t open System Preferences

When I click on System Preferences in the Apple drop down menu, all the headings fade to grey but nothing else happens. After a few seconds when I click on a blank part of the desktop the headings reappear but no System Preferences.

When I open the drop down menu there is a rounded box next to System Preferences that says 1 update, but clicking on it does nothing.

I recently updated from Catalina to Big Sur.

Hardware is iMac Retina 5k 27-inch late 2015

Any advice welcome.