[Other Accounts] I cannot add a Gmail account through the "Add External Email" feature.
Please follow these steps to troubleshoot the issue:

1. Verify if POP is enabled for your Gmail account.
Go to Gmail POP activation

2. Use App Password: If you have enabled two-step verification, log in with an app password. (Google's login policy has changed from May 30, 2022, and you can no longer register external emails using only your username and password.)
Go to Google Announcement
Set up 2-Step Verification for your Google account
Generate an App Password for your Google account
3. If the problem persists despite the above instructions, please visit the 'Allow Access to My Google Account' page and follow the provided guidance.

For more detailed assistance, please consult the Google Help resources.