Unfortunately, it's not possible to create sub-tasks inside of sub-tasks.
However, there is a feature request open for this to be implemented, you can access it in the below link.
Considering the fact that there is no estimated time or decision for this feature to be implemented, adding yourself will ensure you're proactively informed of progress and updates going forward in accordance to our Implementation of New Features Policy.
We are sorry for the impact this lack of feature causes in your work.
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