DIAGNOSIS That's the 'filter fault' indicator. The machine thinks the PECO-Filter is either missing, not seated correctly, or has reached its official end-of-life and needs replacement.
FIX First, power cycle it. Unplug the unit from the wall for a full 60 seconds. While it's unplugged, open the front door and physically remove the PECO-Filter, then re-insert it firmly, making sure it's clicked all the way into the slots at the top and bottom. Close the door, plug it back in, and wait. The startup sequence takes a minute—if the filter is recognized, the red light should go away.
IF THAT DIDN'T WORK If the red light persists, the filter's NFC chip (the little circuit on the side) might not be communicating. Try gently cleaning the gold contacts on the filter and the corresponding pins inside the unit with a dry cotton swab. If it's still red, and you've had the filter for over a year of continuous use, the unit has likely locked it out. You'll need a new official PECO-Filter.
HEADS UP Don't bother with third-party or 'reset' chips you might see online. The Air Pro uses an authenticated system, and unofficial filters will keep the red light on. The unit is essentially a brick without a recognized filter.