It's a very common mistake in non-native speakers. In very colloquial language (teen-speak, for instance) you might use the noun as an adjective - "it wasn't a fun evening", though that would be really wrong in any formal or semi-formal context. (I mark it as "wrong" in my students' writing, and don't say it myself, but they nearly all use it between themselves. The language changes and the young speak a different version!)