Most likely that means you have not run all the previous cells in the notebook. Anytime you close and reopen the notebook, you need to rerun everything to recreate the runtime state of the notebook. Try “Cell → Run All Above” and then run your test again.
To explain this a bit more, the question is where is “np” defined? It is in the very first “import” cell in the notebook, right? Please go find that cell and read it to understand what it does. Then run it. Then try your test again.
If you are new to DLS, it’s worth taking a look at the DLS FAQ Thread. There are many topics there, including one about “name not defined”. Please have a look and it explains the same thing that I explained above.
BTW all this was also explained in the lecture in Week 2 called “A Quick Introduction to the Jupyter Notebooks”. If you are struggling with the mechanics of the Jupyter notebooks, that might be worth another look (or a first look as the case may be). 