DIAGNOSIS The sensor on the water reservoir isn't making proper contact with the base. It's a common issue where the contacts get gunked up or the reservoir isn't seated just right.
FIX Unplug the brewer. Lift the reservoir straight up and off. Look at the bottom of the reservoir and the contacts on the brewer base. Wipe both sets of metal contacts with a dry cotton swab or cloth. Dry the entire bottom of the reservoir. Place it back on the base, pressing down firmly until you hear a distinct click. Plug it back in. The 'add water' light should clear.
IF THAT DIDN'T WORK The float inside the reservoir might be stuck. Turn the reservoir upside down and shake it firmly (with the lid on tight). You should hear the float moving freely. Reinstall.
HEADS UP Never submerge the brewer base. Only the reservoir, brew basket, and carafe are dishwasher safe. Forcing the reservoir on can bend the pins.