Diagnosis
This is a battery communication or overload fault. The tool's electronics see an issue and won't allow start-up.
How to Fix It
First, remove the battery. Check the battery terminals on both the tool and battery for dirt or debris. Clean with compressed air or a dry cloth. Reinstall the battery firmly. Pull the trigger. If the red light persists, try a different, known-good DeWalt 20V battery. A weak or faulty battery is the most common cause.
If That Didn't Work
The tool's electronic switch module may have failed. This requires professional repair. The DCB205 or DCB203 battery models are known to sometimes have communication faults with this tool.
Heads Up
Never use compressed air to clean while the battery is installed. You can force debris into the connections.