DIAGNOSIS The descale reminder wasn't properly reset after the cycle. The machine thinks you still need to descale, or the reset sequence wasn't completed correctly.
FIX To reset the reminder light, the machine must be in standby mode (on but not brewing). Press and hold the 'Strong' button and the '12oz' button together for 5 seconds. The descale light should turn off. You might need to try this a couple times—the button combo timing is picky.
IF THAT DIDN'T WORK Perform a full system reset. Unplug the machine. Press and hold the physical Power button for 15 seconds. Plug it back in while still holding the button, wait 10 more seconds, then release. Let the machine go through its full startup. This often clears stuck error states.
HEADS UP The descale reminder is triggered by a simple brew counter, not an actual sensor detecting scale. If you use filtered water or descale very frequently, the light will still come on based on time/usage. You can just reset it using the button combo without actually descaling again.