If statement is not working due to truth value of a Series is ambiguous

I have got the value of mean that is .0004 from the program given below.

dff = pd.read_csv("I01.csv")
avg = dff['A'].rolling(3).mean()
dff['I_mean'] = avg
window = []
listpos = 0 
for datapoint in dff.A:
    mean = dff.I_mean[listpos] 
    print(mean)
    if (datapoint < mean) and (len(window) < 1):
        listpos += 1

After simulation the program if statement is not working. It shows error.

  A    0.0004
  dtype: float64

ValueError                                Traceback (most recent call last)
<ipython-input-12-49a6c73fdd23> in <module>()
     74     mean = dff.I_mean[listpos] 
     75     print(mean)
---> 76     if (datapoint < mean) & (len(window) < 1):
     77         listpos += 1

~\AppData\Roaming\Python\Python37\site-packages\pandas\core\generic.py in __nonzero__(self)
   1328     def __nonzero__(self):
   1329         raise ValueError(
-> 1330             f"The truth value of a {type(self).__name__} is ambiguous. "
   1331             "Use a.empty, a.bool(), a.item(), a.any() or a.all()."
   1332         )


ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().

.0004 is the mean value. If I put this mean value (.0004) directly on the code then my program shows no error. Why it's happening? What's wrong in the code?