Very weird bug in assignment 3 - pandas-practice-assignment

See the below error, I must say a very strange one. All the questions, from 1-13, are marked as Pass and still I have failed the assignment, and the reason is because of some other weird 9 questions. Please check this asap!

Question 1: FAIL
The value of ‘sample_mean’ is incorrect
NotebookRuntimeError : name ‘sample_mean’ is not defined

Question 2: FAIL
The value of ‘population_mean’ is incorrect
NotebookRuntimeError : name ‘population_mean’ is not defined

Question 3: FAIL
Incorrect solution
NotebookRuntimeError : name ‘std’ is not defined

Question 4: FAIL
The value of ‘z_statistic’ is incorrect
NotebookRuntimeError : name ‘z_statistic’ is not defined

Question 5: FAIL
The value of ‘test_type’ is incorrect
NotebookRuntimeError : name ‘test_type’ is not defined

Question 6: FAIL
The value of ‘p_value’ is incorrect
NotebookRuntimeError : name ‘p_value’ is not defined

Question 7: FAIL
The value of ‘is_significant’ is incorrect
NotebookRuntimeError : name ‘is_significant’ is not defined

Question 8: FAIL
The value of ‘uplift’ is incorrect
NotebookRuntimeError : name ‘uplift’ is not defined

Question 9: FAIL
The value of ‘confidence_level’ is incorrect
NotebookRuntimeError : name ‘confidence_level’ is not defined

Q1: PASS
Q2: PASS
Q3: PASS
Q4: PASS
Q5: PASS
Q6: PASS
Q7: PASS
Q8: PASS
Q9: PASS
Q10: PASS
Q11: PASS
Q12: PASS
Q13: PASS