If 2-step verification (2SV) is active, you must enter a device setup password into the [Password] field. Check your mobile phone for a text message about your account.
If you see this message it means that 2SV has been switched on and the device must be authorised for you to sign-in. You should have received a text message letting you know that the account setting was changed. If you did not receive a text and did not enable 2SV, either switch the setting off from another device or contact us as soon as possible.
If you did switch on 2SV you need to authorise your device. Find out how to do this here.