You filed this question under “AI For Everyone”, but I think you are asking about DLS. I will move the thread for you by using the little “edit pencil” on the title.
That error just means you haven’t run all the previous cells in the notebook. Try “Cell → Run All Above” and then run the test again. Note that you have to do that every time you reopen or restart the notebook. It’s also documented on the DLS FAQ Thread, which is worth a look on general principles.