Now you have 3 options:
1 Option. Plug your U2F device in, click Submit, and press the button on the device.
2 Option. In case you don't have your U2F device you can use your mobile authenticator app. Click Enter a two-factor code from your phone. Find out How to log in using your phone here.
3 Option. In case you have neither U2F device nor mobile authenticator (phone) you can use backup (recovery) codes and enter a one-time secret reserve key.