Your filter is spent. The red light next to REPLACE blinks when 0% of the filter life remains.
Replace the filter immediately. After installing the new filter, reset the indicator. Press and hold the STATUS button. If you installed a blue Brita LONGLAST filter, hold for 6 seconds until the green light next to LONGLAST FILTER blinks three times. If you installed a white standard filter, hold for 2 seconds until the green light next to STANDARD FILTER blinks three times.
If the light still blinks red after resetting, you may have selected the wrong filter mode. Repeat the reset process, holding for the correct duration for your filter type.
Heads up: The indicator's battery is sealed and non-replaceable. After several years, it will stop working, but you can still use the pitcher. Just remember to change the filter every 120 gallons for LONGLAST or 40 gallons for standard.