There is no add Issue type available in my JIRA currently. Only Work type available.

[Set Chou Kee]

I would like to create a new project top track customer complaints.
I realize that there is no longer Add issue type available in the Setup.
I can only add Work type. 
What is the difference between them?

[Piyush Annadate _ACE Pune_]

Hello @Set Chou Kee ,
Welcome to the Atlassian Community !

Work item is new term for issue in JIRA, JPD, JSM. Atlassian recently did a rename of issue as work item.
That's the same name; nothing changed - only UI change.

Ref this article  about the change and why the change.