Cannot unlock Users & Groups settings
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:


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


Next steps

If you still can't unlock the Users &amp; 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&#39;t have administrator account on my mac
From Standard to Administrator