Syntax error when finding duplicates in Numbers

Someone please please help me! I'm trying to find duplicates in Numbers and I'm 100% sure that I'm entering the right formula. Because, I'm watching a "how to" video on Youtube and the guy on the video writes the same formula and it works. But when I do the same thing it says syntax error. By the way, it doesn't matter what formula I enter, I always get syntax error. Someone please help me or I'm gonna throw this computer out of the window.

[Re-Titled by Moderator]