Check if your project has any sub-task ticket types. Your jira/project admins can help you you on this. i.e. Go to project settings --> Issue type If there are any sub-task issue types added to the project, you can see it with a label sub-task.
Sub-tasks should be created from a parent ticket. i.e Open a parent ticket and you can see "Create sub-task"
Please note: You cannot create a sub-task under a sub-task.
