How do you make stair treads with an open riser stop squeaking? The treads are connected to the stringer with brackets.
You could try tighting the brackets. If the brackets are screwed on unscrew remove thread and apply some adhesive like F-26 or Liquid Nail then reinstall. If that's not possible you could screw or nail thru top of thread directly to the riser. This old school trick might be an option, squirting a small amount of either baby powder or graphite between the tread and riser. I'm guessing the stairs are made of wood.