DIAGNOSIS That's the machine's way of telling you the water reservoir isn't seated correctly. It's a sensor issue, not a brewing fault.
FIX Lift the reservoir straight up and off. Look at the bottom—you'll see a little silicone gasket. Make sure it's clean and seated flat. Then, align the reservoir over the posts and press it down firmly and evenly until you hear a distinct *click*. The display should clear.
IF THAT DIDN'T WORK Check the black float inside the reservoir. If it's stuck in the up position, the machine thinks it's empty. Gently push it down and move it up and down a few times to free it.
HEADS UP Don't just wiggle the reservoir. The click is key. If you don't hear it, the sensor pins aren't making contact.