If the two lines appear in exactly the same place on every image, with or without flash, the most likely cause is a hardware fault in the camera—usually the image sensor or the electronics that read data from the sensor. This is not likely to be caused by the lens, lighting, or a camera setting.

There isn’t a user fix for this. The best next step is to have Nikon service evaluate the camera, especially since it was recently purchased. If possible, provide an original NEF/RAW file when contacting support, as that can help confirm the issue.