Do you realize that you do not need anything but the return line?
# maximum = nums
# for num in nums:
# maximum += num
# if maximum == max(nums) and max(nums) > maximum:
None of the preceding code does anything to help execute the function. In fact, the if condition:
if maximum == max(nums) and max(nums) > maximum:
… can never be satisfied. Look at it: under what values of maximum and nums could it return True?
Please realize: In a beginning coding course, it is very common to be assigned tasks for which there already exists a function. Unless the instructor specifically asks you to do so, simply plugging in that pre-existing function (in this case, max()), although it may get a green check when run through the automated grader, does not satisfy the assignment.
Didn’t your teachers at one time ask you to learn long division and multiplication despite the fact that you had access to a calculator?