You didn’t specify what screens you are trying to control, but please note that POWERDEVIL_NO_DDCUTIL=1 disables the *DDC control in Powerdevil. If your screen is controlled by another mechanism - such as a laptop backlight ACPI control - this will not prevent Powerdevil from controlling the screen brightness.
Also, you didn’t specify if you did or didn’t but you also need to logout after the change, for it to take effect.