If you can't set it even after you've entered the code displayed in real time on the authentication app, try uninstalling the app and then reinstalling it.
■ How to set up Google Authenticator
1. After logging in, click “Security Settings”.
2. Under "Google authentication", click the "Settings" (or "Reset") button.
3. Install and launch the "Google Authenticator" app and enter your "Google Authenticator Code".
4. Enter the code you receive.
5. Click the "Confirm" button.
Note: if the time setting of the smartphone or the time setting of the two-step authentication application is off, authentication may fail, so please check this as well.