DIAGNOSIS: The machine thinks it still needs descaling. This usually happens if the descaling procedure wasn't completed correctly—the machine needs to see the whole cycle, start to finish, to reset the counter.
FIX: You have to run the official descaling mode exactly by the book. Fill the water tank with the Nespresso descaling solution (or a citric acid solution) diluted with water as instructed. With the machine on, press the button three times within two seconds. The light will blink. Put a container under the spout. Press the button to start. It will run liquid out, then stop. You must then refill the tank with fresh water, press the button again, and let it run a full rinse cycle until it automatically stops. Only then will the light turn off.
IF THAT DIDN'T WORK: If you completed the full cycle and the light is still on, the machine's internal descaling sensor or timer might be stuck. Try unplugging the machine for a full 5 minutes to let it do a hard reset. Plug it back in. If the light is still solid orange, a factory reset might be needed (procedure varies by model year).
HEADS UP: Don't interrupt the descaling cycle. If you unplug it or turn it off mid-cycle, the counter won't reset and the light will stay on. You'll have to start the whole, tedious process over from the beginning.