Start with a visual inspection. Look at the wires, had a rear sensor get cut from road debris. Then follow the wires to the connector points, not hard just trace the wire back. Disconnect and inspect. if it has corrosion ( Looks like crap) clean and reassemble with a water repellant electrical grease.
Next clean the tone ring ( That toothy ring near the hub) dirt and grime can get in there and foul the sensor. You have a tone ring on each wheel on the street side ( as apposed to the curb side) Clean both.
If a sensor is loose, the spacing between the tone ring and the sensor is out of spec and you will get an ABS light.
Test ride, just have to go down the driveway. Problem solved ride happy, if not you most likely have a sensor that has gone bad or connections in and about the ABS unit in the rear of the bike. ( Under the rear fender) ps Stupid place to put electronics Ma Vic.
The wiring in these bikes is not great when it come to water intrusion. Swimming thru deep water and flogging thru downpours will play havoc on your system.
I use a good quality Dielectric Grease in all of my connectors.
One last thought, Check the battery connections for tightness and corrosion. Yes this is stupid but the battery will cause all sorts of funky electrical issues.