What's wrong with this code?

[perfringo]

As buran pointed out - never use sum as variable name. I would add - never use Python as typing machine. You should see the pattern in ISBN check number calculation: every second number in ISBN is multiplied with 3. So no need to do all this manual labor:

>>> s = '978-0-306-40615'
>>> nums = (int(num) for chunk in s.split('-') for num in chunk)           # remove '-' and convert into int
>>> calculated = (v  if i % 2 == 0 else v * 3 for i, v in enumerate(nums)) # keep in mind that Python is 0-indexed
>>> last = 10 - sum(calculated) % 10
>>> last
7