On a MacBook with macOS Catalina, how do I unlock the Keyboard-en.plist file?
By default the system drive is mounted read-only in Catalina. You'll need to remount it as writable in order to edit the file. 

You can do that by opening the Terminal in Recovery Mode (after disabling SIP) and then running the following command:

sudo mount -uw /


Note that after a reboot, the system drive will be back to its previous read-only mode.