DIAGNOSIS The machine has hit its programmed cleaning reminder interval (roughly every 3 months of use). It's a timer-based alert, not a sensor.
FIX The light will not turn off until you complete a cleaning cycle. Fill the reservoir with a 50/50 mix of white vinegar and water. Place the empty carafe on the warming plate. Press and hold the CLEAN button for 3 seconds to start the cycle. It will run for about 30 minutes. After it finishes, discard the vinegar water and run 2-3 water-only cycles to rinse. The clean light will reset.
IF THAT DIDN'T WORK The button membrane might be stuck. Unplug the machine. Dampen a cloth with isopropyl alcohol and firmly wipe around the CLEAN button to clean any sticky residue. Let it dry completely before plugging back in.
HEADS UP Do not add coffee grounds or a brew basket during the clean cycle. The vinegar solution runs through the internal system to descale it. Using a commercial descaler is fine, but vinegar is the manufacturer-recommended method.