The backup account works only when there is a data connection available because it validates your authenticity. If no data connection is available, then the Google account verification won't work properly. Try pressing the Power button until the menu appears. Additionally, enable data connection and then input the Google account. If this solution doesn't work, you will need to hard reset the phone.
Turn it off and then press Volume Up + Home + Power button until the Samsung logo appears then release the Power. The recovery menu will be displayed. Choose Wipe Data/Factory Reset using Volume Keys and confirm the operation with Power.