Hello and welcome to the forums.
I'm not sure why the script isn't running, there may be some information in the /var/log/pm-suspend.log file to indicate why its failing.
As an alternative, you could directly echo brightness values into the backlight interface files. To do so, you would echo a value between 0 and the contents of the file max_brightness to the brightness file. The backlight interface files are located at /sys/class/backlight and there may be more than one, so you'll need to go into each of the directories and test it out.
For example, if you have an acpi_video0 interface (/sys/class/backlight/acpi_video0) you would have a number of files in that directory. Check the contents of max_brightness and echo a value between 0 and max_brightness to the brightness file:
Once you find the correct interface and correct value to set the brightness, change the line:
echo 5 | sudo tee /sys/class/backlight/acpi_video0/brightness
COMMAND="xbacklight -set 0"
...where X is the value and INTERFACE is the name of the interface (directory).
echo X > /sys/class/backlight/INTERFACE/brightness
As an example:
echo 5 > /sys/class/backlight/acpi_video0/brightness