That looks even more awkward than the red elbow. As mentioned above, it may help if the pressure is first relieved by removing the pump's power lines & spinning the starter. If it hadn't been removed recently, a little lube (I find silicone or lanolin spray works) in the teeth of the QD helps. Once you have if off, lube those teeth again from the inside for next time. You push the fuel line in while pulling the grey collar back. There is no click or other tactile feedback to tell you how well you're going. It doesn't take much pressure or effort. Just pull the collar back, perhaps wiggle it slightly, and all of a sudden there's a little left over fuel on your fingers & it's off. Once it's off, and you've done what's needed, it is an idea to practice doing it again several times, so that next time your fingers know what to expect. There's not much in it, and your fingers will end up knowing more than the rest of you does, and it'll work every time. And remember, it's a Guzzi, so if you struggle & struggle, and it's still not working, take a rest, a cuppa, a beer, a glass of red, a run around the block, whatever. When you come back to it, it usually works first go.
