How to add underscore between "non" or "no" and the word followed by it using python?

You could use the pandas "apply" method as follows.

import pandas as pd

def func(s):
    tokens = s.split()
    i = 0
    while i<len(tokens):
        if tokens[i] in ["no","non"] and i < len(tokens)-1:
            tokens[i] = f"{tokens[i]}_{tokens[i+1]}"
            tokens.pop(i+1)
        i+=1
    return ' '.join(tokens)
            

df = pd.DataFrame({'id':[1,2], "text":["no damage car", "non damage car"]})
df["text"] = df["text"].apply(func)

The resulting dataframe df:

   id            text
0   1   no_damage car
1   2  non_damage car

Granted, the function being applied could be made "nicer" with the help of regular expressions.