While we have a lot of fun slang terms here, a slightly more formal term (that could still be used casually!) would be [**Incompetent**](http://www.oed.com/view/Entry/93712?redirectedFrom=incompetent#eid):

> **a.** Of inadequate ability or fitness; not having the requisite capacity or qualification; incapable.
>
> **B.** An incompetent person.