DIAGNOSIS The continuous red light is the 'tank full' indicator. The unit has a safety float that stops operation when it thinks the internal bucket is full, even if you've just emptied it.
FIX This is almost always the float switch being stuck. Power the unit off and unplug it. Remove the water tank. Look inside the unit where the tank sits—you'll see a small orange or white plastic float on a hinge. Gently press it up and down several times to make sure it moves freely. Reinstall the tank, making sure it's seated perfectly flat and level, then plug the unit back in.
IF THAT DIDN'T WORK If the red light persists, the unit might be on an uneven surface. The slightest tilt can trigger the float. Place a small level on top of the unit and adjust its feet or move it to a completely flat, hard floor. Carpets can cause this.
HEADS UP Never try to run the unit with the tank removed to bypass this—you'll cause a major water leak. The safety system is there for a reason.