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"
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.