Program is correct, but Grading is 0

TF1, W2 assignment, I am facing the problem.
When I opened the submission, It is showing
There was a problem compiling the code from your notebook.
Details: name ‘os’ is not defined

And is graded as ZEro. How to resiolve.

Just do
import os
See if this works
Thanks and Regards,
Mayank Ghogale