Where in System Preferences do I find how to enable system extensions? A friend of mine has a Mid 2012 MacBook Air running Catalina, and he's trying to get the Android emulator BlueStacks running on it.  He gets an error saying

System extension blocked.
Enable the extension from Security &amp; Privacy System Preferences pane by clicking 'Allow' button and BlueStacks will launch again.

I looked in that pane, and couldn't find anything like that.  I'm obviously missing something here but I don't know what!
A screenshot of the error is attached:
Open System Preferences 
Open the Security &amp; Privacy preference pane 
Choose the General tab 
Click the lock to make changes.  (In versions of macOS as applicable.)
Click Allow in the bottom-right.
This Allow button will only show if there is a system extension that is pending approval.