This left-most light is a diagnostic indicator. When blinking, the blink pattern ("flash code") can be interpreted to determine the particular malfunction.
When this light is not illuminated at all, the most common reason is that the scooter battery charger is plugged into the scooter or the scooter battery while the battery is on the scooter. For safety reasons, the scooter is designed not to be operable if the battery charger is plugged into the scooter.
For Scout scooters, the second-most common reason for that light being out and the scooter not working is because of an issue with the contacts connecting the front and rear halves of the scooter. Search "front and back" in the Help Center for an article on how to resolve this.