If you have a machine with an ICE-bank fridge, that has built in sparkling water, this feature is great.
Built into the fridge is a carbonation unit, that can dispense roughly 1,5 liters in one dispense.
The Co2 system will always try and maintain pressure inside the tank, which is why it starts carbonating after every dispense.
If, by selecting sparkling water and only gas comes out, this means the tank is empty. The reason for it being empty comes down to two causes:
Water inlet is closed, no water is flowing through the system.
Verify this by trying to dispense a normal, cold water. If it this is not flowing either, it's the water inlet that is closed off or perhaps blocked. Check you shutoff valve, kinks on the hose and connections to and from the filters.
If the cold water is flowing,
The Fridge is turned off
If cold water is flowing, but the gas is coming out, there is a good chance your fridge is simply turned off by accident. The reasons being, that the inlet valve cannot allow more water into the tank, causing only the Co2 to flow.