Extract text from a series into new columns

I’ve tries the following code with the following error message:

AttributeError: ‘DataFrame’ object has no attribute ‘str’

I’m referencing a string series, then why am i getting a "Dataframe" error?

df['PO'] =  df['extract'].str.extract(r"(?:^(?=[A-Z\d/-]+$)|\bPO\W*)([A-Z\d/-]+)")

>Solution :

Problem is with duplicated columns names, so if select one column extract get all columns in DataFrame.

print (df['extract'])

For deduplicted is simpliest set columns names or some another solution:

df.columns= ['a','b']