Extracting email id gives error
You may have missing data (e.g. np.nans) in your COMMENTS field. This will throw an error in your email_extract function.
Try to filter the problematic rows out, e.g. with 

filtered_data = data[~data.COMMENTS.isna()] #purge problematic comments


and applying the email extraction to the now "clean" column:

filtered_data.COMMENTS.apply(lambda x: email_extract(x))