Diagnosis
This is almost always a sensor issue. The float inside the water tank isn't triggering properly, so the machine thinks it's empty.
How to Fix It
First, unplug the unit. Then, take the water reservoir out. You'll see a small plastic float on the side. Gently move it up and down with your finger a few times to make sure it's not stuck. While you're there, wipe the sensor area (the little plastic post it slides on) with a clean cloth. Put the reservoir back in, plug it in, and that red light should go out.
If That Didn't Work
If the light's still on, the reservoir might not be seated all the way. Push down firmly on the back left corner of the reservoir until you hear a distinct click. Sometimes it needs a good shove.
Heads Up
Don't ignore this light and try to run it. If the sensor's faulty, it could run the pump dry and burn it out. Always make sure that light is off before you start a cycle.