A 320 Class

The ELAC (Elevator and aileron computer) as the name suggests, control the elevators and the ailerons. There are two ELACs in the Airbus A320. ELAC 1 and ELAC 2. In normal operations, ELAC 1 primarily controls the ailerons and ELAC 2 controls the elevator and the horizontal stabiliser. In flight, these two computers are inter changeable. That is, if for example ELAC 1 were to fail, the aileron control is transferred to ELAC 2. The aileron control transfer is fairly straight forward and simple. However, this is not the case when transferring elevator controls.
The ELAC 2 controls the left and the right elevator surfaces through the yellow and the green hydraulic system. If ELAC 2 fails, the elevator control is passed onto the ELAC 1 which controls the elevator through the blue hydraulic system. Unlike ELAC 2 there is not two hydraulic systems assisting ELAC 1, but one system which is the blue hydraulic circuit. If the aircraft is dispatched with ELAC 2 inoperative, the ELAC 1 controls the elevator through the blue system. If the blue system fails, ELAC 1 cannot transfer control to other computers such as SEC (Spoiler elevator computer). ELAC 1, unlike ELAC 2 does not transfer elevator control to the SECs in a hydraulic failure.
Elevator and THS control of the A320.
Moreover, ELAC 2 is very critical if the aircraft were to go into an emergency electrical configuration, where the RAT (Ram air turbine) extends and powers an emergency generator which powers the blue hydraulic system. If you do not have a functioning ELAC 2 and ELAC 1 is only available, you might lose elevator control until the RAT fires up and start running the generator. The RAT extension takes a bit of time and losing pitch control even for a few seconds can be catastrophic.
Why is it not allowed to dispatch an Airbus A320 with an inoperative ELAC 2?
In case of loss of both the generators in flight, power supply to the primary flight computers ELAC1, ELAC2, SEC1 and SEC2 is disrupted (SEC3 is supplied but it does not have a pitch channel). The green and yellow hydraulic systems are still pressurised since the engine driven pumps are operative but the blue system is temporarily lost.
Hence, before the RAT (Ram Air Turbine) extends for about 8 seconds,
ELAC1 and SEC1 are powered by battery 1 and ELAC2 and THS motor 1 are supplied by battery 2.
This enables the ELAC2 to control both the elevators with the green and yellow servocontrols while the THS is operated with the motor 1 controlled by ELAC 2 again.
(ELAC1 can not control the elevators since blue hydraulic pressure is not available neither can SEC1 or SEC2 for similar reasons.)
With gear extension or possibility of a RAT stall, pitch authority is still retained by ELAC2 which makes it an absolute no go item.

In flight control sysThere are 2 ELAC 1&2, Why ELAC 2 is no go item ????

Green light means the fuel quantity selected from cockpit ...
When light is off means refueling also completed.