That message usually means the mirror or shutter mechanism is out of alignment or jammed after the drop. Sometimes pressing the shutter again can let it cycle back into place, which is why Nikon shows that prompt.

If the error does not clear, the safest recommendation is to have the camera inspected by an authorized service center. Even if the body looks fine externally, a fall can cause internal mechanical damage that isn’t visible.

One community reply described manually nudging the mirror, but that is risky and can cause further damage, so it’s not a safe general recommendation.

In short: try the shutter release once or twice as prompted, but if the error remains, stop using the camera and get it serviced.