On all four wheels their will be a sensor and 'sensing' plate. First step is to clean these all up with a parts cleaner, and try again. If one of the sensors cant pick up a signal from the plate, then this will cause a fault. If no luck, inspect the wiring to and from the sensors, for obvious damage/short circuits. Hopefully this would have sorted it, otherwise their could be a faulty abs sensor

It has to be fixed then cleared with a scaner