Crack the bleeder on the rear caliper and push the pedal. If fluid squirts out, the problem is in the caliper, and is likely a stuck piston or two.

if no fluid comes out, and you’ve abs, it’s likely failure of the abs module.

Ok. I ride constantly. Had this happen on my 2011 Limited. It was winter and weather was nasty so I was not washing it and it sat up between rides. I lost my rear brake and could stand on the brake pedal. I could not activate the ABS. and was pushing very hard. I even went on a sand covered road and stood on that brake pedal. So I went home and parked it waiting for a good day that I could bring it in. I go in the garage one day and see something on my right side rear wheel. Oil dripping on the tire coming off the rotor coming from the wheel bearing. Like I said I could not wash due to weather and that’s how I noticed the oil. The wheel bearing was gone and leaking grease or the oil from the grease breaking down. It had dripped onto the rotor and that’s why it was not breaking. Brakes were good just rotor covered in oil so it was not any friction to stop.