Hi Dong H Jeon,
Thank you for reaching out to Microsoft Community Support.
According to this issue, I have reviewed and there are other threads mentioning about this:
GMAIL no longer accepts standard POP connections as of April 2025 | Microsoft Community Hub
Help with Outlook 365 and Gmail (following OAUTH changes) - Microsoft Community
You could try to generate an app password from your POP connection setup and then replace the current password with the app password to see if it works.
If you have New Outlook, you could try to add that account using POP connection as well.
Thank you again and your understanding is highly appreciated.
Sincerely,
Austin H - MSFT | Microsoft Community Support Specialist