If you attempt to do a task and see the following message:
Thanks for your hard work!
You have completed all of the work that you can for this task. Please check back again in the future. We are always adding new jobs and would love to have your help.
You can find more tasks by going back to the list of available tasks.
When you see this message it means that that task is currently not available, even if you have never worked on it or it says it's still available on the main page. A bit confusing we know... but all you can do is skip this task and try another... you can also try it later, it may be available then.