DIAGNOSIS The machine's internal timer has hit the descale reminder cycle. It's not an error, just a maintenance nag light based on the number of brew cycles.
FIX To reset it, you need to initiate a cleaning cycle. With the machine off, press and hold both the 'Clean' button and the 'On/Off' button simultaneously for about 5 seconds. The 'Clean' light should stop flashing and stay solid, then you can run a vinegar-water descaling cycle as described in the other answer. After that cycle completes, the light should go out.
IF THAT DIDN'T WORK If the light keeps flashing after a descale cycle, the reset sequence might not have registered. Try unplugging the machine for a full 5 minutes. This sometimes clears the reminder counter.
HEADS UP This light is just a timer, not a sensor. It can't actually detect scale. If you use filtered water, you can ignore the light and just reset it—but you should still descale every few months.