That means you must have accidentally removed some of the comment lines that turn out to contain information that the grader uses. You can get a clean copy of the notebook to see what that cell should look like by using topic B.5 on the MLS FAQ Thread. Search for the string UNQ_C1 in the clean notebook to find the relevant graded code cell.
Just generally speaking, it should only be necessary to change things in the “YOUR CODE HERE” sections. It’s not illegal to change other things, but you need to exercise appropriate care. 