The most common cause of this issue is time synchronisation in the Google Authenticator app.
To resolve this issue and to make sure that it’s set to the correct time:
- Go to the main menu in the Google Authenticator app
- Tap 'More' and choose 'Settings'
- Tap 'Time correction for codes'
- Tap 'Sync now'
You will see that the time has been synced on the next screen, and the system will start accepting verification codes.