finishedGamesGroupedByWinner sounds like it's a dictionary or list or something. You should check its type.
Python can't divide a list of numbers, it can only divide numbers. Example:
>>> 3.0 / 4.0
0.75
>>> [2.0, 3.0, 4.0] / [5.0, 6.0, 7.0]
Traceback (most recent call last):
File "<pyshell#1>", line 1, in <module>
[2.0, 3.0, 4.0] / [5.0, 6.0, 7.0]
TypeError: unsupported operand type(s) for /: 'list' and 'list'
It sounds like you need to loop through the players and divide each one.