Try the following on your Mac in order, until (hopefully) resolved:
1. Create a New User Account
○ System Preferences > Accounts
○ Click "+" to add a new account.
○ Log off, and then, log back on with the new user account.
○ Try connecting to a Wi-Fi network with your Mac's AirPort.
○ If this resolves the problem, there may be something amiss with your current profile and will require further troubleshooting.
○ If this does not resolve the problem, go ahead and log back into your normal account and go on to the next step.
2. Delete AirPort Keychain Entries
○ Launch the "Keychain Access" application, located in Applications/Utilities.
○ In the left-side window, select "login" under Keychains and "Passwords" under Category.
○ Click on the "Kind" filter at the top and look for any "AirPort network password" entries...and delete them.
○ Close Keychain Access.
3. Delete & Add Back Preferred Network(s)
○ System Preferences > Network > Wi-Fi > Advanced > Wi-Fi tab
§ Delete all entries under "Preferred Networks."
§ Click on OK.
○ System Preferences > Network > Wi-Fi > Advanced > Wi-Fi tab
§ Add the preferred network(s) using the "+" button.
§ Click on OK.
○ Restart or log out, and then, back in.
§ If it works, then you are done.
§ If not, go on to the next step.
4. Move System Configuration Files
○ Quit any applications that are currently using Wi-Fi.
○ Turn off Wi-Fi from the Wi-Fi icon on the macOS menu bar.
○ Open Finder > Go > Go To Folder
§ In the "Go to the folder" window, enter:/Library/Preferences/SystemConfiguration/
§ Click on Go
○ Locate and select the following files:Note: Files are macOS version specific, not all the following files are used in each version.
§ com.apple.airport.preferences.plist
§ com.apple.network.eapolclient.configuration.plist
§ com.apple.wifi.message-tracer.plist
§ NetworkInterfaces.plist
§ preferences.plist
○ Move any matching files to the macOS Desktop. You can place them all in a folder as a backup. The "missing" files will be rebuilt after restarting the Mac.
○ Reboot the Mac.
○ Re-enable Wi-Fi from the Wi-Fi icon on the macOS menu bar.
Try accessing the Internet again.