NameError: global name 'cb' is not defined

NameError: global name 'cb' is not defined

I am trying to make a program that calculates the biggest number of
objects(nuggets) you can't get with packages of 6-9-20 (I am fairly new to
python, i trying using global and nonlocal but it doesn't work either).
def nuggets(n):
x = 6
y = 9
z = 20
for i in range(0,n//x+1):
for j in range(0,n//y+1):
for k in range(0,n//z+1):
if i*x + j*y + k*z == n:
return [i,j,k]
return None
def cant_buy(n):
seq=0
for i in range(n):
p=nuggets(i)
if type(p)== list:
seq+=1
elif type(p)== None:
cb=i
seq=0
return cb
Then this error appears: Traceback (most recent call last): File "", line
1, in cant_buy(12) File "C:\Python33\OCW 6.00\ps2a.py", line 22, in
cant_buy return cb NameError: global name 'cb' is not defined
What is wrong? I defined it at the elif statement.