How to delete a Draft in JIRA

Hello @Hyochul Sung 

Welcome to the Atlassian community.

Based on the screen image you provided you are working with Jira Data Center or Jira Server (versus the SaaS product "Jira Cloud").

Jira doesn't support the concept of a "draft" of an item. Once you provide the information and click the Create button the item is created.

If you have created an item in error you may delete it only if you have been granted permission to delete items. This permission is often limited to administrators because once deleted the item cannot be recovered.

If you have been granted that permission you would find the Delete option under the More button which is next to the Assign button. It would be at the bottom of the options list.