How to fix gmail SMTP debug error please log in via your web browser on phpmailler?

There are two ways to resolve this, and only one may work, depending on how you're accessing Google.

The first method is to authorize access for your IP or client machine using the https://accounts.google.com/DisplayUnlockCaptcha link. That can resolve authentication issues on client devices, like mobile or desktop apps. I would test this first, because it results in a lower overall decrease in account security.

If the above link doesn't work, it's because the session is being initiated by an app or device that is not associated with your particular location. Examples include:

An app that uses a remote server to retrieve data, like a web site or, in my case, other Google servers
A company mail server fetching mail on your behalf
In all such cases you have to use the https://www.google.com/settings/security/lesssecureapps link referenced above.

TLDR; check the captcha link first, and if it doesn't work, try the other one and enable less secure apps.