The result obtained from pandas series operation cannot get assigned to a variable

I can't reproduce your problem but you can probably solve it simply by improving your code.

1. Make a function that parse one sentence
2. Apply this function to all elements in the column review

Basically, your code should look like this:

def parse_sentence(sentence):
    sentence = sentence.lower()
    sentence = re.sub(pattern, '', sentence)
    ...
    return sentence

clean_reviews = df["review"].map(parse_sentence)
print(clean_review)