IT WORKED! I have an old Macbook and somehow lost my Gmail in apple mail. Tried everything and finally went to Google Security and 2 Step Authorization then added the APP password. Back to Mail and added an Internet account and the app password it worked! All my mail is back, thank you to whoever suggested that in the community. Hours and hours wasted until I found this.
MacBook Pro 15″
I encountered the same error and could not login at all, even with an "app password." It works perfectly fine on my iPhone and despite it showing up as a suggested account in System Settings -> Internet Accounts, I still wasn't able to sign in.
The culprit? AdGuard. I had to temporarily disable it while I added the account.
AdGuard's transparent network filter, I suspect was blocking one of the sites responsible for this process... which is also a bit worrisome.
No need to create an app password as many suggested or add the IMAP account manually because Apple Mail is an oAuth-compatible app that should work fine normally with or without 2-factor authentication.
Error 400 for Gmail in apple mail on Macbook
