My Mac doesn’t help an admin user?! Help! I have two Mac Book Pro's laptop with me.

In one of the laptop, I am getting the following popup prompting when I try to change the login options.

It is prompting me like this:



In my another Mac Book Pro, it's simply showing the prompt like this. So I guess, it should show the same prompt message in first Mac Book Pro too.



How to get administrator name to login into the first laptop?  I tried with possible usernames &amp; password (I have never changed my password) that I am aware of but they are not working.


  How to find/get the administrator name back so that I can log in with my password.
Go to your terminal and type the following command in: 

dscacheutil -q group -a name admin


This will list the admin username.