Hello Karthik,
Within your issue click on the more drop down, then select create sub-task. If there are sub-task issue types related to the project you can create one from there.
On the issue you want to create a sub-task for, select More > Sub-Task. Here is the documentation for it, https://confluence.atlassian.com/jirasoftwarecloud/creating-issues-and-sub-tasks-764478439.html#Creatingissuesandsub-tasks-CreateSubtaskCreatingasub-task
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