There is not a "character monologue" of Scout presented in the novel, other than in the form of her narration of the book itself. If you've been given the assignment to write a character monologue for Scout, this is more of a creative writing assignment than something you can "find" in the novel.
A "monologue" is a term used in drama to denote one character speaking a particularly lengthy speech. It is very often done alone on stage and usually reveals the characters inner thoughts and emotions about an event in the play. If you are writing a character monologue for Scout, depending on the specifics of the requirement, my advice would be two choose one particularly dramatic or humorous scene in the book (such as the mob scene or Scout's first day of school) and have Scout present what happened along with her reactions. Remember, her narration of the book comes from her adult point-of-view, but maintains her childhood memories of events, including her innocence and ignorance of many things. A creative monologue might choose either a fully adult Scout's presentation, or the young Scout's interpretation without the eyes (and voice) of wisdom behind them.