Q: Unable to Lock System Preferences

Hi there Tom Johnson!

It sounds like your System Preferences isn't securing itself by locking when you click the lock. I would start by repairing the permissions on your hard drive with this article:

Repair disk permissions with Disk Utility

1. Open Disk Utility, which is in the Utilities folder of your Applications folder.
2. Select the startup disk from the list of volumes.
3. Click the First Aid tab.
4. To check permissions, click Verify Disk Permissions. To repair permissions, click Repair Disk Permissions.

Disk Utility checks a file's permissions only if the file has a corresponding receipt in /var/db/receipts. The receipt tells Disk Utility what the permissions should be. Not all installers include a receipt with the files they install.

If the issue persists then I would boot to safe mode and see if it will lock and unlock there. Safe mode flushes some system level caches and repairs the startup disk. If it does, then restart as normal and test the issue again. If it does not restart as normal and go to the next step:

Starting up in safe mode

Follow these steps to start up into safe mode.

1. Start or restart your Mac. 
2. Immediately after you hear the startup sound, press and hold the Shift key.
3. Release the Shift key when you see the Apple logo appear on the screen.

After the Apple logo appears, it might take longer than usual to reach the login screen or your desktop. This is because your Mac performs a directory check of your startup disk as part of safe mode.

To leave safe mode, restart your computer without pressing any keys during startup.

From: Try safe mode if your Mac doesn't finish starting up

Thank you for using Apple Support Communities, let us know how it goes!

Thanks for the update Tom! I'm glad you got the issue resolved!