Why is this a fragment sentence? [closed]
Technically, it's not a sentence fragment. It's in the imperative mood (i.e., it's a command), but it doesn't make much sense that somebody would ever say that.
"Studies indicate that antibacterial soaps may wash away useful bacteria", on the other hand, is a well-formed sentence that makes sense. I'm guessing that your (textbook, teacher, ?) wants that.