Getting a Nan ouput from KendalTau method for detecting trend in monthly based sales

Question

I get a Nan result when I used KendalTau method via Python.

Here's my python code:

Data imported from Excel

  Month     BG
0       1  15626
1       2  16266
2       3  20615
3       4  17273
4       5  20002
5       6  42527
6       7  71436
7       8  58615
8       9  38760
9      10  34851
10     11  30580
11     12  36650

tau, p_value = kendalltau("Month" , "BG")

print(tau)
print(p_value)
print() 
if p_value < 0.05:
    print("The correlation is statistically significant.")
else:
    print("The correlation is not statistically significant.")

!!! I imported data from my excel table before code that's that !!!

I was trying to detect if there is a trend in monthly based sales data via Python. I tried KendallTau and expect numerical results instead of Nan value ?