add missing admin account on Macbook Air 2017

Hi getbackto,

Given that you're running High Sierra, you're in luck. You can use Single User Mode on your Mac to resolve this issue.

Single User Mode is a special startup mode on your Mac. Instead of loading the desktop, your Mac starts up to the command line with superuser (root) privileges. If you want to restrict access to this powerful startup mode, make sure FileVault is enabled on your Mac afterwards.

These steps will use a few commands to delete a special file (.AppleSetupDone) and restart your Mac. If your Mac can't find .AppleSetupDone, it assumes that you just installed macOS, and runs the Setup Assistant. From there, you can create an additional administrator account by choosing not to migrate any data.

1. Restart your Mac while holding down Command-S. Release the keys when white text appears on a black screen.
2. If prompted to log in, do so. (If this happens, FileVault is enabled.) The console window will return after you enter your password.
3. Wait for the text to stop scrolling. Then, press Enter (Return).
4. Type these commands in order, exactly as shown. Press Enter (Return) after typing each command:
   1. mount -uw /
   2. rm /var/db/.AppleSetupDone
   3. launchctl reboot
5. Your Mac should immediately restart. If prompted to enter your password, do so to continue.
6. The macOS Setup Assistant should appear. Continue until you're prompted to migrate data.
7. Choose NOT to migrate data, then click Continue.
8. You should now be asked to create a new administrator account. Choose a name that is different from your existing account.
9. When the Setup Assistant is completed, the desktop should appear. Open System Preferences -> Users and Groups.
10. Click the padlock in the bottom left corner, then authenticate with your temporary admin credentials.
11. Select your real user account, then check the box next to "Allow user to administer this computer".
12. Restart your Mac.
13. Log in as your real user account. Your admin permissions should be restored now.
14. If you want, delete the temporary account you made earlier.

Step 1 launches you into Single User Mode.

If you're still not able to get into Single User Mode by holding down Command-S while you restart, check if a firmware password is enabled on your Mac:

Restart while holding down the Option key.

• If you see a selection of startup disks, choose Macintosh HD and hit Enter (Return) to start up normally. Your Mac doesn't have a firmware password set.
• If you see a padlock with a password field instead, your Mac has a firmware password enabled, and won't be able to start up in Single User Mode. If you don't know the firmware password, press and hold the power button (Touch ID) until your Mac powers off.

Hi getbackto,

Just to clarify, are you able to log into your user account, but it doesn't have administrator privileges? Or are you not able to log into your Mac at all?

Also, are you currently running macOS High Sierra (10.13)?