Go to the issue and use the tools menu to click "create sub-task"
If it is not there then you either do not have "create issue" permission in the project, or your project does not have any sub-task types in the issue type scheme. Log in as an admin to fix both of those.
I have login with admin and still, I'm not able to create subtask.
I can't find the subtask option and I have given permission in this projectScreenshot from 2017-03-20 17-38-42.png
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It's under the tools menu to the right of the issue. If it's not, then please re-read my answer.
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