An ink cartridge is expended or not installed.
Replace the ink cartridge with a new one.
If the light is still on after you have replaced the cartridge, it may not be installed correctly. Reinstall the ink cartridge so that it clicks into place.
See Replacing an Expended Ink Cartridge
The currently installed ink cartridge cannot be used with the printer.
Replace the incompatible cartridge with an appropriate ink cartridge.
See Replacing an Ink Cartridge Before It Is Expended
Incorrectly installed ink cartridge
An ink cartridge is not installed correctly.
Reinsert the ink cartridge.
See The error light is flashing or is on after you replace the cartridge