I can't add google mail account, it said the account is already signed in

pm7856 wrote:

How to add email account to mail in MacBook Air

see if there is anything here—

gmail issues—

use the steps from Google

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

https://support.google.com/a/answer/9003945#oauth

Do you see the account in System Preferences->Internet Accounts? Maybe you already have that, eg, for contacts, then all you'd have to do is also check the box for mail.