There are unfinished tasks.

The hierarchy cannot be completed until all other tasks in the project have been completed.

If you are seeing this message, then there are still open tasks that need to approved.

To see what tasks are still open and find out who on your team needs to complete the project follow these steps:

1. Go to the tasks tab in the project.
2. Select the Inactive checkbox and click Apply Filter. 
3. All open tasks and the team member assigned to that task will show up in the list.