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 everyone! My name is Sarah Schuster, and I'm a Customer Success Manager in Atlassian specializing in Jira Software Cloud. Over the next few weeks I will be posting discussion topics (8 total) to ...
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