Yes, at Gent says, your code is correct. This is a pretty straightforward function, of course. So also as Gent says, the problem must be something else wrong with your notebook. Do the other functions get graded successfully? What does the grader actually say under the “Show grader output” link for the euclidean function? That might give us some clue.
Gent’s point about changing the name of the file is another thing to check. If you are working in a renamed copy and click “Submit”, the grader doesn’t grade the notebook you are running: it grades the one opened by the “Launch Lab” link. So perhaps the grader is seeing different code than what you show above.
One other thing I notice is that the format of the “UNQ_C2” comment in your code is different than what I see. So did you perhaps copy this notebook from an older version of this course? That might also cause grader problems. In that case, Gent’s suggestion of getting a fresh copy and then “copy/pasting” over just your code from the “YOUR CODE HERE” sections is definitely the next thing to try. Here’s a thread about how to do that.