DIAGNOSIS: The filter life indicator has hit zero and is locking you out. This is a mandatory reminder, not a mechanical failure.
FIX: You need to manually reset the filter timer. On the side of the unit, there's a small pinhole next to the filter icon. Use a paperclip to press and hold the button inside for about 5 seconds. The screen should clear and go back to the main display. You don't necessarily need a new filter right that second, but the reminder will come back on schedule.
IF THAT DIDN'T WORK: The unit might think the filter isn't seated. Turn it off, unplug it, and remove the filter cartridge. Check the white plastic filter head inside the machine for any debris or a stuck O-ring, wipe it clean, and reinstall the filter firmly.
HEADS UP: The filter timer is based on time, not actual water quality. If you use it heavily, you might want to change the filter before the timer goes off. Resetting it without changing the filter just silences the nag.