Q: Accidentally deleted WiFi service on my MacBook 2012 and it won't let me add it back. Hit the   button and nothing happened.

You will likely need a Restart to set it right. Then try to add it back.

Then you may need an SMC Reset:

Resetting the System Management Controller (SMC) on your Mac - Apple Support

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How do I ask Grant a question?  

If it's about this subject, post it right here.

If it is completely unrelated, click new and start a new topic.

If you cut-and-paste my (Blue-text) name into the message, I get a reminder I have been mentioned in a particular thread.

Triple-click anywhere in the line below on this page to select it:

/Library/Preferences/SystemConfiguration

Right-click or control-click the line and select

          Services ▹ Open

from the contextual menu.* A folder named "SystemConfiguration" should open. Move the contents of that folder to the Trash, except the file with this name:

          com.apple.Boot.plist

Leave that file where it is.

You may be prompted for your administrator login password. Restart the computer and test.

If there's no change, restore each of the items you deleted from a backup, overwriting the one that may have been created in its place. Otherwise, you may need to recreate some of your settings in the Network and Energy Saverpreference panes.

*If you don't see the contextual menu item, copy the selected text to the Clipboard by pressing the key combination command-C. In the Finder, select

          Go ▹ Go to Folder...

from the menu bar and paste into the box that opens by pressing command-V. You may not see what you pasted because a line break is included. Press return.