Why does it give me that error?
Because your first parameter you pass to the loop
function is None but your function is expecting an callable object, which None object isn't.
Therefore you have to pass the callable-object which is in your case the hi
function object.
def hi():
print 'hi'
def loop(f, n): #f repeats n times
if n<=0:
return
else:
f()
loop(f, n-1)
loop(hi, 5)