When you have a task in "Available tasks" list, it means that it may be available not only to you, but to some other users as well. Another user can take this task and start working on it.
If you want to work on the the task from "Available tasks" list, first assign this task to self.
How to assign available task to self:
Choose the task in "Available tasks" list and click "Assign to me" link button. The task will move to "My tasks" list.
2. Another option: click on the task in "Available tasks" list to access task details page. Use green button "Assign to me" in the top right corner.
3. When you assign task to self, it is no longer available to other users and you can start working on the task
Please note: Task data form becomes visible only when you assign task to self.
Next article - How to work on assigned task