DIAGNOSIS The machine has hit its recommended cleaning cycle interval (usually after something like 24-48 hours of cumulative operation) and locked you out until you run the cleaning cycle.
FIX You have to run the clean cycle now. Unplug the unit, empty all ice and water. Fill the reservoir with a mix of clean water and white vinegar ( gives the ratio, often 1:1 or a cup of vinegar to a full tank). Plug it back in, press and hold the "Clean" button for 3 seconds until it beeps or the light stays solid. Let it run its full 10-15 minute cycle, then drain, rinse twice with fresh water, and refill to use normally.
IF THAT DIDN'T WORK If the clean light continues to flash after a completed cycle, the internal timer might need a reset. Unplug the machine for a full hour, then plug it back in. This sometimes clears the reminder.
HEADS UP Don't ignore the clean light. Mineral buildup from tap water will eventually clog the lines and evaporator, leading to permanent damage and weird-tasting ice.