Within Teamwork Projects, a task list is automatically marked as complete once each of the individual tasks in the list have been completed.
To mark a task as complete, you can select the tick mark to the left of the task title/assignee.
Once you have completed all tasks within the list, it will mark it as complete. It will then move to the Completed Task Lists area on the left pane of the tasks area.
The completed task lists area is useful to review all completed tasks/task lists on a project as it retains all of these in one place.
For more information, see: Pinning a task list to keep it open/active