DIAGNOSIS: The machine is detecting it doesn't have enough water to safely start the heating cycle. This usually means the internal reservoir isn't full, even if the external tank looks okay.
FIX: Turn the unit off. Unplug it. Remove the external reservoir and pour it out. Lift the entire machine and tilt it gently side-to-side—you might hear water sloshing inside the internal tank. Set it back down, plug it in, and press and hold the power button for 10 seconds. Refill the external reservoir and let it sit for a few minutes to allow the internal tank to fill completely before trying to heat again.
IF THAT DIDN'T WORK: The float sensor inside the internal tank might be stuck. With the unit unplugged and the external tank removed, use a toothpick or cotton swab to gently probe and wiggle the small plastic float you can see inside the tank opening. Don't force it.
HEADS UP: Never try to run a heating cycle without the external reservoir seated correctly and full. It's designed to fail-safe with the red light to prevent the heating element from burning out.