You linked your tasks, but now they aren't linked correctly.
Are you sure that you linked your tasks? Placing tasks in a hierarchical order does not automatically link the tasks.
Are you trying to link to a summary task? Summary tasks cannot be successors in finish-to-finish or start-to-finish task dependencies. You can link summary tasks only in finish-to-start or start-to-start task dependencies. Similarly, summary tasks cannot be predecessors in start-to-start or start-to-finish task dependencies. You can link from summary tasks only in finish-to-start or finish-to-finish task dependencies.
Did you recently rearrange the tasks in your outlined schedule? After you rearrange the linked tasks in your outlined schedule, the task dependencies that you set previously may no longer be relevant, and you may need to update them.