Why can't I reinstall my gmail.com account to the apple mail program?

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

Didier wrote:

rasmee209: of course I read the Error message,but I have no clue how to correct the situation and how comply with Google's 0Auth 2.0 policy. Can you help me please?

start here https://support.google.com/mail/?hl=en

This older Apple thread may also shed light on the problem.

High Sierra Mail problems with Google - Apple Community

It appears Google no longer supports the Apple Mail app included in older macOS versions, and you are running 10.13.

Didier wrote:

Suddenly the Apple Mail would not download any mail from the gmail.com account. I checked my account on the Google Mail site and all mail was still there. I tried to re-install this account in the Internet Accounts app but get the Error 400 message all the time

Strangely I have a second gmail account that operates normally. Also, I can see my gmail on the iphone.

I am using iMac with OS 10.13.6 High Sierra

If anybody has a solution please replay to this post.

https://discussions.apple.com/content/attachment/a66aeadf-a257-4f7e-be74-51e27b1b1504

Did your read the screenshot you posted, it tells you exactly why you can't access your account.