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"RE: Nikon D5100-Subject is to dark; cannot adjust exposure;Use the flash"
As Alan said, it's not an error message, but a warning. Your camera will still take the photos. Just as a test I actually managed to even overexpose a shot while the message was showing (yeah I exaggerated every setting possible). I think what it also might mean is that the subject has no lighter tones (don't cameras try to expose at 18% grey?) and so the camera literally can't see enough to decide how to expose the image.
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