The reason for the confusion is that "fun" can also be used as an adjective, which describes something as enjoyable.
Here are examples of "fun" as a noun:
* "We had a lot of fun at the party."
* "The kids are having fun playing in the park."
Here are examples of "fun" as an adjective:
* "It was a fun movie."
* "We had a fun day at the beach."
So, while "fun" can function as both a noun and an adjective, it's not always a noun. The context usually makes it clear which way it's being used.