A Nikon preset WB reading can fail for a few common reasons:

• Exposure is off: If the reference shot is too dark or too bright, the camera may reject it. Make sure exposure is reasonable before taking the preset reading.
• Light is too spectrally unbalanced: Very warm light, such as candlelight or other strongly colored light sources, may be beyond what the camera wants to correct automatically. In those cases, preset WB may return “no good.” A manual Kelvin setting can work better, even if it leaves a slight warm cast.
• Framing can matter: One user found better results by not filling the frame entirely with the card, but instead including some surrounding tones/colors.

To improve success:

1. Use normal exposure settings.
2. Defocus if needed so the target is evenly sampled.
3. Try the gray/white target filling most of the frame, but if it fails, step back or zoom out slightly.
4. If the light is extremely warm or odd-colored, use a manual Kelvin setting instead of preset WB.