In addition to Warren's response, the reason why this is not possible is due to the hierarchy of the application as Project > Issue > Sub-task
You can also have multiples stories linked to an epic and have sub-tasks on stories, but this is more like an Agile concept, so if you want to have additional information about that, I'd recommend you to take a look at the article below:
Hey Community mates! Claire here from the Software Product Marketing team. We all know software development changes rapidly, and it's often tough to keep up. But from our research, we've found the h...
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