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

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

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.

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

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

I'm having same issue, but mine is specific to a corporate google account... just trying to add a calendar. Works fine on my other Macs, but it isn't working on this new Mac with latest Mojave release (10.14.6). I click "OPEN SAFARI" and nothing happens. (Screenshot below)

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).

Well I sure hope some clever engineers are following the user forums (I sure do in our product line). Normally I'm a pretty considerate and calm person. But this thing got me really on the wrong foot and my mood is way below cellar. Of course I would have preferred to present a way to work around. But so far nothing has helped. No reboot. Delete account then try to create it again. Second best guess is to remove the google account and create a non-google account with all the corresponding draw-backs. Once things work again we could do the reverse. Welcome to the 21st century.

I'm having the same problem. I switched to 2 stage ID for Google and now when I try to add Google in Mail i get "Google requires completing authentication in Safari". I hit the Open Safari button and get nothing. Using Mohave 10.14.6 and Safari 13.0

Now this is ridiculous and severely annoying! This is basically all I can say without getting insulting.

FWIW, this is a users forum so "getting insulting" is pointless.

Safari13 is not good !

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

Indeed, problems seem mainly in Mojave

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

I guess for the record -- I'm having the same issue on my iMac after installing the 10.14.6 update. My laptop is working fine, though.

Issue is fixed with 13.01. update.