When you're flagged it says exactly this
Quote:Sorry - you cannot work on this task
The author of this task has flagged your work as being unsatisfactory. If you believe your work was wrongly flagged please contact us using the "Send Feedback" link in the upper right hand corner.
What you got was only because either there was no more tasks to do, a bug, or there were no more tasks available. Try refreshing in a few minutes. I'm not an expert though lol