Thank you for this remark, Xinyue Wu.
What happens is that there are many TED videos, but there are also many Chinese language volunteers ready to make Chinese subtitles: thus the tasks for doing so tend to be picked fast.
However, if you act quickly when you get notifications of new videos available for subtitling, you'll probably find a Chinese one you can pick.
When I clicked "task" the website showed that no videos for me to work on. And no matter what I searched it showed "no result"