Google requires completing authentication is Safari - CAN'T DO IT AFTER UPDATE

I fixed it! Emil Ng on the same topic thread on Gmail Help provided this:

https://support.google.com/mail/thread/14841442?hl=en

I managed to fix this by 

1. remove my Google account from Internet Accounts in MacOS System Preferences
2. remove access to MacOS in my Google account.
3. 
   
   1. go into Google Account
   2. go into Security
   3. find "Third-party apps with account access"
   4. click on "Manage third-party access"
   5. click on "macOS ... Has access to Gmail, Google Calender..."
   6. click on REMOVE ACCESS button
4. restart MacOS
5. add Google account to Internet Accounts in MacOS System Preferences

They just released Safari 13.0.1 that addresses this issue. I had to fully restart my machine after installing the update (wouldn't work at first), but then it finally worked. This had me pulling my hair out for a while - glad they put out a quick fix (quick for me anyway, since I just ran into the problem today).

FWIW, the problem is Safari 13.0 on Mojave. Safari 12.1.2 works. So far I haven't seen a fix. In the meantime, a different email client should work for Gmail accounts and/or use webmail until it's resolved.

The Google Calendar and Account issues are related to Safari 13 in combination with MacOS 10.14.6:

Yesterday I have upgraded to Safari 13. Since then I get the message:

"Google requires completing authentication is Safari"

In the Internet-Accounts, my Google Account exists.

But as soon as I click on it, that message appears again. With the options "Cancel" and "Open Safari". Cancel does not help.

For "Open Safari":

• If Safari is already open, nothing happens, no new window and no login dialog.
• If Safari was not open, Safari will start, but only with the previously opened tabs.

When I open the Calendar app, the Google Calendar has a warning triangle and does not update my calendar data.

Unfortunately, I do not find a way to authorise again. And the Google Calendar can no longer be used with the calendar app, since Safari 13.

As consequence I have rolled-back to the status before Safari 13 - with the help of Time Machine. With Safari 12.1.2 everything works fine again. Of course, this is not a satisfying solution. Apple will have to fix this issue in Safari 13.

This issue is also related to that discussion:

https://discussions.apple.com/thread/250658935?page=1

this issue has been resolved now with the new update of safari 13.0.1. Just update safari and try adding the account again it works perfect.

FYI, Safari just released an update(9/25/19) that will resolve the google authentication issue to add to mail

Apple just today released a new version of Safari — Version 13.0.1 — which solves this problem.

Issue is fixed with 13.01. update.

Check the Safari update released yesterday ... download and restart and voila it works again

Safari13 is not good !

You can try getting into gmail with your webbrowser. If that does not work with Safari13, try Firefox.

Yes, you need to restart your machine after installing

Not to add insult to injury but my Gmail accounts were already established and work even after the Safari 13.0 debacle. It's also strange that Safari 13.0 works in High Sierra and Catalina.

You can send a bug report/feedback from: Feedback - Safari - Apple

Workaround: I needed to set up a new gmail account... was getting the same message... so I did it on my iPhone where the Google verification worked fine

13.0.1 Still not working for me. I guess I am going install and use SPARK email client from now on.

Apple I hope you are reading this!

This Safari update worked for me ... while I had to restart twice ....