You didn’t understand the assignment.
Before starting an essay, make absolutely sure you understand the assignment and the type of essay required. If you are not completely certain, ask your instructor or TA. If your school has a writing centre, go to them. At the very least, ask your classmates or friends, just be wary that they may be as confused as you are. Once you get started, it’s important to focus on the work ahead instead of worrying that you’re doing the wrong assignment.
The Efficient Essay provides examples of various types of essays and how to know which one is right for your assignment.