Randomly allot values to the rows in python

I have the following input table (y):

parameter1	parameter2
1	12
2	23
3	66
4	98
5	90
6	14
7	7
8	56
9	1

I would like to randomly alot values from A1 to A9. The output table should look like the following:

parameter1	parameter2	parameter3
1	12	A5
2	23	A2
3	66	A4
4	98	A8
5	90	A3
6	14	A7
7	7	A1
8	56	A9
9	1	A6

n = 9

TGn = round(len(y)/n)
idx = set(y.index // TGn)

y = y.apply(lambda x: x.sample(frac=1,random_state=1234)).reset_index(drop=True)
    
treatment_groups = [f"A{i}" for i in range(1, n+1)]
y['groupAfterRandomization'] = (y.index // TGn).map(dict(zip(idx, treatment_groups)))

I am unable to fill the first row value it prints as NaN. How do I tackle this problem?