Replace this line with your code.
The only error we can see is the indentation on the last line. Remove the indent.
There's a lot of code in the average() function. It can be simplified:
return float(sum(numbers)) / len(numbers)
but no matter. As you progress you will find lots of cool ways to refactor your code. What's important right now is that you getting the outcomes you expect.
There is also the issue that your final
else: statement should be
return "F", not