ou can't sign in to this app because it doesn't comply with Google's OAuth 2.0 policy for keeping apps secure.

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.

Most likely biggest factor is using Other in Mail setup, then using a Browser that doesn’t have gMail PW stored init to get the App PW.

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

https://support.google.com/a/answer/9003945?hl=en#apple-mail&zippy=%2Cset-up-gmail-with-mail-on-a-mac

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…

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

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

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

Possible gMail fix…

https://discussions.apple.com/thread/254806558?answerId=259768998022#259768998022 see Youtube video…

https://www.youtube.com/watch?v=U0WPw2TsTAY

Apple Mail App Not Functioning - Apple Community

https://discussions.apple.com/thread/254931663?answerId=259192691022#259192691022

https://discussions.apple.com/thread/254659965?answerId=258757070022#258757070022