Cannot unlock Users & Group in System Preferences

You're best bet is probably to just boot into macOS Recovery mode and then use the resetpassword command in Terminal to reset your password.

More specifically:

1. Power off your Mac
2. Boot into Recovery Mode by holding down the CommandR keys while powering up your Mac
3. If prompted, select your language and press enter
4. Go to Utilities > Terminal.
5. Enter resetpassword (all one word, lowercase letters)
6. Press Return
7. Select the drive with your account (typically the main drive)
8. Choose your account using Select the User Account.
9. Enter a new password
10. Re-enter the same password
11. Enter a password hint
12. Click Save
13. Ignore the warning about the password changing
14. Click OK.
15. Shutdown your Mac by going to Apple > Shut Down
16. Restart your Mac normally and use the new password to log into your account
17. Once logged in, test to see whether you can unlock the Users & Groups preferences pane

Next steps

If you still can't unlock the Users & Groups preferences pane after the above steps, it may well be that you have no Admin account on your Mac. If so, refer to the following for further info:

• I don't have administrator account on my mac
• From Standard to Administrator

I found a working solution to this on this Apple support community thread, Can't install programs and unlock settings:

OSX apparently needs to regenerate a fresh auth.db from /System/Library/Security/authorization.plist.

This can be triggered in terminal with the following command: sudo mv /var/db/auth.db /var/db/auth.db.old

After rebooting the system everything works as it should be.