column_data = [row[1] for row in reader] IndexError: list index out of range

This error occurs when you are trying to access an element of a list that does not exist. In this case, it seems that you are trying to access the second element of a row in a CSV file, but the row does not have at least two elements.

To fix this error, you should check the contents of your CSV file and make sure that each row has at least two elements before trying to access them. You can also add a check in your code to handle cases where a row does not have enough elements.