The system preferences cannot unlock the user and groups and hence cannot make changes to the computer. I am using OSX El Capitan.

You need to be logged into an Administrator account to modify System Preferences. If you have an Admin account you can use, then log into that account to make the changes.

look at the very bottom of the window. Like many System Preferences screens, you must "Unlock the lock to make changes".

You do this by clicking on the lock and entering your Admin username and password in the dialog box that appears.

If you are the only User, you ARE the Admin. Unlock the lock by clicking on the padlock Icon and proceed.

If you don't have an Admin account then you can do one of the following:

From Standard to Administrator Account

How to Create a New User Account if You Cannot Access Your Admin Account

Reboot the computer. At the chime press and hold down the COMMAND-S keys to start into Single-user Mode. You will boot to a black screen with scrolling white text, release the keys and wait for it finish.

Enter each of the following lines at the command prompt and press RETURN:

mount -uw /

rm /var/db/.applesetupdone

reboot

After you reboot the computer you will be taken to the Setup Assistant. When you get to the point where you are asked to setup your new user account, create a new admin account. Be sure to name this user account something different than the admin user account that already exists on the system. When you finish the Setup Assistant, it will automatically log you into the new account.

You can transfer your files from the old account to the new one. See: Transferring files from one User Account to another. Once you have moved all your files into your new account, you can delete the old account using Accounts or Users & Groups preferences.

[Copied from another post by Kappy.]

Please take these steps to restore administrator privileges to your account. This somewhat tedious procedure is only necessary if you've confirmed that no working administrator account exists on the system.

If you don't already have a current backup of all your data, you must back up before taking any of the steps below. Ask if you need guidance. You won't need the backup unless something goes wrong—which is always possible.

Step 1

Start up in Recovery mode. The OS X Utilities screen will appear.

Step 2

Take this step only if you use FileVault 2. Launch Disk Utility, then select the icon of the FileVault startup volume ("Macintosh HD," unless you gave it a different name.) It will be nested below another drive icon. Select Unlock from the File menu and enter your login password when prompted. Then quit Disk Utility to be returned to the main screen.

Step 3

Select

Utilities ▹ Terminal

from the menu bar. In the window that opens, type this:

resetp

Press the tab key. The partial command you typed will automatically be completed to this:

resetpassword

Press return. A Reset Password window opens. Select your startup volume if not already selected. Pull down the menu labeled

Select the user account

and select

System Administrator (root)

Follow the prompts to set a password. It's safest to choose a password that includes only the characters a-z, A-Z, and 0-9. I suggest you write down the password. If you don't write it down and forget it, you'll have to start over from Step 1.

Select

 ▹ Restart...

from the menu bar.

Step 4

This step, like Step 2, applies only if you use FileVault. Log in as usual, then select

 ▹ Log Out...

from the menu bar, or press the key combination shift-command-Q. Don't restart. You'll be returned to the login screen.

Step 5

At the login screen, click Other... Enter "root" (without the quotes) in the Name field, and enter the password you set in Step 3 in the Password field. You should now be logged in as root. This is a potentially dangerous condition. Do nothing while logged in as root except as indicated below. You'll be fine as long as you don't deviate from the plan.

Open the Users & Groups preference pane. Select your usual administrator account in the list of users and check the box marked

Allow user to administer this computer

You'll be prompted to restart. Do that and log in as yourself—not as root. Your administrator status should now be restored.

Step 6 (optional, but recommended)

Follow the instructions in this support article under the heading "Disable the root user." You must authenticate in Directory Utility as "root" with the password you set in Step 3. Authenticating as another administrator won't work.

Credit for this idea to ASC member wessongroup.