How to create a subtask and then subtasks to the parent subtask



Hi there,
I’m trying to automate the creation of a lot of Subtasks, as in the title.
For this I use as a Trigger a Spreadsheet with the all of the Task ID’s i want to update, then update this Task. Then I want to create a subtask, and around eight subtasks to this subtask.
However, if I use as a custom value for parent task the ID created in step three(creating the parent subtask), I get an error message: “We had trouble sending your test through. Error:
parent: Unknown object” So it does not find the subtask created before.
What am I missing? Or is this not even possible?
Thanks a lot in advance!