Error 400 invalid request

Mac's Mail App is not as modern as iOS's Mail app, Many eMail providers now require a separate App Password different from your eMail password…

If it's an IMAP account, delete it in Sys Prefs Internet Accounts then use "Other" instead of gMail choice to add it.

It might just need an App Password separate from the gMail password…

Apple mail will not let me add a GMail ac… - Apple Community

https://support.google.com/mail/answer/185833?hl=en-GB

Apple Mail won't sign in to Google account - Apple Community

my iMac has OS Yosemite 10.10.5 Does this… - Apple Community

my iMac has OS Yosemite 10.10.5 Does this… - Apple Community

gmail on mail Version 11.5 (3445.9.1) - Apple Community

gmail on mail Version 11.5 (3445.9.1) - Apple Community

Note from PrestoWebDesign…

https://discussions.apple.com/thread/253701684?answerId=257151754022#257151754022

Trying to add Gmail account to Apple Mail… - Apple Community

https://blog.google/technology/safety-security/the-beginning-of-the-end-of-the-password/

Hi all! I have had the same error after updating to 17.2. I quickly learned that the authentication was being run in safari's private browsing. I was able to go into the settings of safari and disable the advanced tracking and fingerprinting protection and was successful. Go into your safari settings, go to the advanced tab and toggle your "use advanced tracking and fingerprinting protection" feature off. Once you are able to authenticate, toggle it right back on for security reasons. Hope this helps!!

👍