When an error happens deep inside functions that call other functions, Python prints a traceback: the chain of calls that led to the error. Run this and open "Python's message" in the error box:

def average(numbers):
    return sum(numbers) / len(numbers)

def report(name, marks):
    print(name, "averaged", average(marks))

report("Sam", [70, 80])
report("Alex", [])

Traceback (most recent call last):
  File "main.py", line 8, in <module>
    report("Alex", [])
  File "main.py", line 5, in report
    print(name, "averaged", average(marks))
  File "main.py", line 2, in average
    return sum(numbers) / len(numbers)
ZeroDivisionError: division by zero

Read it from the bottom up:

The last line says what went wrong: ZeroDivisionError.
The frame just above says where: line 2, inside average.
Each frame above that is how it got there: report called average on line 5, and the main program called report on line 8.

The bug is often not on the line that crashed. average is fine for normal lists; the real question is why it was called with an empty one, which the earlier frames tell you.