This is most often caused by entering the incorrect wireless password into the Wii U connection settings.
What to do:
Ensure your Internet connection is working.
If your wireless router or access point is not connected to the Internet, this error code will appear during the connection test.
Check the Internet connection of other devices connected to your wireless router or access point. If other devices do not connect, this indicates an issue with your home network that will need to be resolved prior to your Wii U connecting online.
Ensure you are using the correct password for your Wi-Fi network.
If your wireless router or access point is secured with a security key or password, review this password to ensure you have entered it correctly in the Wii U connection settings. [How to]
Power cycle your home network.
If your wireless router or access point is unresponsive, power cycling this device may allow your Wii U to connect online. [How to]
Enter an alternate DNS into the Wii U settings.
If the DNS settings failed during the connection test, entering an alternative DNS into the Wii U settings may allow you to connect online. [How to]
Still Not Resolved:
If the issue persists, please contact us at 1-800-255-3700. Specially trained representatives will be available to assist you, 6:00am - 7:00pm (PST), seven days a week.