我想通了。
在执行IF语句之前，我必须首先使列行相互匹配。

df = pd.read_csv(Dir)
df = pd.DataFrame(df)

df = pd.merge(df[["Email Invite"]],
              df[['Email Denied']],
              left_on='Email Invite',
              right_on='Email Denied',
              how='left')

这样，DataFrame将如下所示：

Email Invite                    Email Denied                                                                                       
batman@email.com                batman@email.com                       
poisonivy@email.com                          
superman@email.com                         
catgirl@email.com               catgirl@email.com             
joker@email.com                             
supergirl@email.com             supergirl@email.com

我继续我的FOR循环和IF语句：

a = len(df['Email invite'])
aList = []

for i in range(a):
    if df['Email invite'][i] != df['Email Denied'][i]:
        aList.append(df['Email Invite'][i])  

df['Emails Left'] = pd.Series(aList)

现在我有我的额外专栏：

Email Invite                  Email Denied           Emails Left                                                                                      
batman@email.com              batman@email.com       poisonivy@email.com                
poisonivy@email.com                                  superman@email.com  
superman@email.com                                   joker@email.com
catgirl@email.com             catgirl@email.com              
joker@email.com                             
supergirl@email.com           supergirl@email.com

现在我可以将其传输到新的CSV。

df.to_csv("NewInvite.csv", index=False)

现在程序运行良好。

不建议使用列长度不同的 Dataframe 。您必须用NaN或' '或其他东西填充剩余的元素，使它们长度相等。列表在以下情况下会更好：

import pandas as pd

df = pd.DataFrame({'Email Invite': ['batman@email.com', 'poisonivy@email.com', 'superman@email.com','catgirl@email.com' ,'joker@email.com' ,'supergirl@email.com'], 'Email Denied': ['batman@email.com', 'catgirl@email.com' ,'supergirl@email.com', '','','']})

email_invited = list(df['Email Invite'])
email_denied = list(df[df['Email Denied']!='']['Email Denied'])

email_left = [email for email in email_invited if email not in email_denied]
print(email_left)