On a MacBook with macOS Catalina, how do I unlock the Keyboard-en.plist file? File path:   

/System/Library/Input Methods/PressAndHold.app/Contents/PlugIns/PAH_Extension.appex/Contents/Resources/Keyboard-en.plist  


After disabling System Integrity Protection (SIP) in macOS Catalina, the Keyboard-en.plist file still remains locked. Each time I deselect the ✓ checkmark appearing in the "Locked" menu in the titlebar of the opened file, the following message appears:  

"You don't have permissions to unlock the file "Keyboard-en.plist." You can duplicate this document and edit the duplicate.  

My goal is to unlock the original  Keyboard-en.plist file so that I can edit text in the file or unlock the original file so I can replace it with a duplicate file containing edited text.
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.