In general it is not a good idea to modify the given parts of the code that are outside the “YOUR CODE HERE” segments. The w2_unittest module was already imported by earlier code in the notebook:
There are various places in the notebook that also call
w2_unittest functions. Do those work?
One thing to try would be to start with a clean notebook and then just copy over your solution code from the “YOUR CODE HERE” sections. Here’s a thread about how to get a clean copy.