Chart managers can create task grouping. Chart editors and chart viewers are not granted such permission.
To create task grouping you would have to click the button on the top toolbar.
Once clicked, you’ll be able to select the fields you want the tasks to group by.
Task grouping by "Parent" field will gather sub-tasks under their parent task.
Do you have tasks in your project that act as a parent to another tasks? If yes, please don’t forget to include the field Parent to grouping fields. Skipping it might cause the incorrect display for task duration.