Setting screen brightness on external monitor

I’m using a Jetson device with an external monitor, and the brightness keys aren’t working.

  1. journalctl -f gives the following error:

gsd-media-keys[2641]: Failed to set new screen percentage: GDBus.Error:org.gtk.GDBus.UnmappedGError.Quark._gsd_5fpower_5fmanager_5ferror.Code1: No usable backlight could be found!

  1. xrandr works fine to adjust brightness from the terminal.

How can I resolve this?