Error 400 and 'Unable to verify account name or password' when adding email accounts on old MacBook Pro with iOS 10.12.6

Old Macbook iOS 10.12.6. Error 400 when trying to use email bcs of new google security.

I set up an app password for Apple Mail and enter that in password box but get error message "Unable to verify account name or password"

Strangely, after numerous attempts over a few days, one of my emails finally works but can't do the same for the others

[Re-Titled by Moderator]

Original Title: cannot add email accounts