Here's why:
* Subject: A subject is the person, place, or thing that performs the action of the verb. In this phrase, there's no clear indication of *who* had fun.
* Verb: "Had" is the verb in this phrase, but it needs a subject to be complete.
To make it a complete sentence, you could add a subject:
* They had fun.
* We had fun.
* The children had fun.