To work with subtsaks. you need to be in manual sort mode.

To create a subtask, long press the parent task, then the add-event-button at the bottom will change to a add-subtask-button.

To change an existing task to a subtask, position it below its parent, long press the task and press the >-button to ident the task (multi-selection is possible).

Google Tasks no longer supports multi-level subtasks.