You need escape ? by \:

You need escape ? by \:

df['URL'] = df['URL'].str.replace('\?v=', 'ppp')
print (df)
     Id            Title                                                URL
0  Id-1   Bruce Almighty        https://www.***.com/watchppp5VGyTOGxyVA
1  Id-2  Superhero Movie        https://www.***.com/watchppp3BnXz-7-y-o
2  Id-3            Taken        https://www.***.com/watchpppvjbfiOERDYs
3  Id-4      Forest Gump  https://www.***.com/watchpppeJFkCJySHdY&t=...

Series.replace 的另一种解决方案:

df['URL'] = df['URL'].replace('\?v=', 'ppp', regex=True)
print (df)
     Id            Title                                                URL
0  Id-1   Bruce Almighty        https://www.***.com/watchppp5VGyTOGxyVA
1  Id-2  Superhero Movie        https://www.***.com/watchppp3BnXz-7-y-o
2  Id-3            Taken        https://www.***.com/watchpppvjbfiOERDYs
3  Id-4      Forest Gump  https://www.***.com/watchpppeJFkCJySHdY&t=...