The drain alert on an air conditioning unit indicates that water, formed as condensate on the units' evaporator coil and collected in a pan, isn't leaving the unit the way it should be. This feature can be used to monitor water levels in the unit and to avoid overflow.
Unplug the unit to check for a clogged drain tube or a full drain pan. Install a continuously draining room air conditioner with a slight downward slope by slanting it forward in the window to facilitate exterior drainage. Clear the plastic drain tube using a soft brush to remove condensate build up and obstructions. Allow the air conditioning unit to dry completely before turning it back on.