Sorry for the delayed response.. i've looked at this a few times and couldn't come up with anythig. but after looking at it a 4th time, i think i have an idea..
After some brief testing, looks like it's a case sensitivity issue. The names of the default presets are title case, but you have a lowercase c in "16 colors".
If you change that variable definition to "16 Colors" it should work.
Where do you come from?
Which language your Illustrator has?
The preset list names are depend on your language.
to get the (real) names of your presets.
After trying about everything I could think of (which ain't much, ha!), I commented out:
alert('cannot find tracing preset that has been set in the script');
just to see what would happen... and it worked like a charm! Can use upper or lower case. Even my custom preset!
oh, to answer:
Where do you come from? USA
Which language your Illustrator has? English