This light is on because the anti lock brake system is not working.
Have the anti lock system repaired and the light will go out.
About 95% of the time this happens because a wheel sensor has failed. This can be checked with a code scanner. The better code scanners will tell you which wheel.
When any fault is detected in the anti lock brake system it is disabled for your safety. You would not want the anti lock brakes to work on only 3 wheels. This would send you out of control under heavy braking.
Your brakes are now working like standard power brakes without anti lock.
