You can only return from inside a function and not from a loop.
It seems like your return should be outside the while loop, and your complete code should be inside a function.
def func():
N = int(input("enter a positive integer:"))
counter = 1
while (N > 0):
counter = counter * N
N -= 1
return counter # de-indent this 4 spaces to the left.
print func()
And if those codes are not inside a function, then you don't need a return
at all. Just print the value of counter
outside the while loop
.