This is an open discussion prompt, and it requires you to explore the statement and defend it or defend a rebuttal. You are certainly able to claim that the statement is erroneous; however, I would definitely caution against that. Most readers are likely to enter into your essay already believing that Robinson Crusoe is a masterpiece. Your reader is much more likely to be supportive of your statements if you are already starting from the same base opinion. Saying that the story is not a masterpiece is likely to give you an antagonistic reader, and that kind of reader is much more difficult to convince of your argument.

I don't know the exact writing requirements that you have been given to explore the prompt, but I do recommend having more than only one supporting argument. If the piece is a masterpiece, you should be able to discuss more than a single reason. Support the masterpiece claim by discussion the themes that the book explores. Another angle to pursue is to discuss how the novel creates and develops characters.

A third option for your essay is focus on the cultural and/or societal impact of the novel. The book was (and still is) a hugely popular book and could be defended as a masterpiece simply because of how exciting the book is and how it was able to get so many readers in such a short period of time. This angle is a bit dangerous though, because being hugely popular doesn't necessarily guarantee that a piece of literature should be called a masterpiece. Take Twilight as an example. Hugely popular. Hardly a literary masterpiece.

Focus on how the novel was able to tap into the spirit of the era (zeitgeist) in which it was written. The book was able to tell a story that still explored popular issues like religion, world commerce, philosophy, and socioeconomic classes.