In order to access Nintendo eShop and other online functions of Wii U, you must perform a system update after connecting your console to the Internet.
To perform a system update, first please ensure that your Wii U console is connected to the Internet.
You can perform a system update by opening System Settings from the Wii U Menu, and navigating to the System Update option.
You can even download data for the system update in the background while you play Wii U games. When the data has finished downloading, return to the Wii U Menu and follow the on-screen instructions to complete the update process.
For more information, please visit the Wii U support section.