The white flashing line is normal. It means the tuner is in "Idle Mode". When the tuner is on, but not being used, it will display a white flashing line on the screen, to remind you that it is still on. Once you begin to tune, the white line should disappear, and the tuner should begin registering notes. Once you're finished tuning, the white flashing line should re-appear, if the tuner remains on.
Please clip the tuner onto your instrument and begin tuning - does the tuner then register notes? If so, the tuner is functioning as intended! If it does not begin registering notes, please contact us