Try linking not connected to your wifi network using your cell data and see if that works. 
At least for me and I assume @anon35356645 , itās an Android problem. Did it all, WiFi, Cell, Work WiFi, in-lawās WiFi, different networks (on Project Fi), nothing. Now to figure out why itās an issue.
iOS Device, worked in a second.
Sounds like an app update issue then, not component/HA issue.
Iām getting the same maddening endless loop with the āAccount linking failedā message. 2 Days ago I added a few new switches to my HA instance and wanted to add them to my Google Assistant. I unlinked and havenāt been able to get it to link since. Iāve tried different browsers on my phone, tried clearing cache, tried from my WiFi and Data, nothing. This has been working fine for a month or so, I donāt know if an update to HA or GA broke it. Iāve downgraded to 0.61.1 and I get the same behavior. I have an Android phone, so maybe Iāll try an old iPad and see if that works.
no same error (on android)
well thatās ridiculousā¦same here. Tried on an old iPad and though one of the Google auth pages said ānullā it redirected and successfully linked my accountā¦
This definitely seems to be a Google issue and not a HA issue.
I also forgot about the request_sync service, so now I wonāt need to unlink and link my account when I make changes, just call that service and the updates will show up in the Google Home app within a few seconds.
@techwithjake thanks a ton man, I seriously spent hours trying to get this working again.
Hopefully Google fixes whatever is causing this.
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Glad to hear it worked for you too, @Bottleface .
Iām gonna keep digging around to see if I can find a fix for everyone else but currently the iOS workaround seems to be the answer.
Definitely use the resync option, itās a life saver. Although, something weird happened with it which is what caused all this in the first place for me lol
If your setup is otherwise working and all you want to do is add new devices from HA, you can also say āHey google, sync my devicesā, and sheāll go through a list of the connected accounts sheās syncing āOK, syncing ecobee and Home Assistantā in my case. And then go to the app and voila! New devices waiting to be added to rooms.
Getting the sync service working from HA was a headache in itself lol. Asking google is much easier.
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I also get the linking failed error while setting up google home support for the first time.
use an iphone/ipad
Had similar issues as others above. Mine starting with trying to Sync which kept failing. The Google Cloud logs seem to indicate an OAUTH failure - No idea why or how it suddenly ran into OAUTH failures.
Anyway, I had to unlink and link. Which showed the Account link failed error but then went on to load all my devices in. Thankfully, I donāt have a lot of devices exposed so wasnāt a major problem adding them back into rooms.
It worked for me, using an iphone. Thatās really strange. Nevertheless, itās seems really unuseful to me at the moment. As sensors can not be queried.
Same here and wife upset as she canāt control light and heating anymoreā¦
Here it started with sync not working. So unlink yesterday and tried to link sinceā¦ Have even created new project in developer console but same resultā¦ Unlucky me we have no iPhoneā¦
I would say lucky you for not having an iPhone
I found I couldnāt sync my devices today. To rectify this, I simply ran through the gactions command in CLI to relink the project. Afterwards, I was able to sync and re-add the instance to my Google Home app.
gactions update --action_package project.json --project [project-name]
Well this isnāt the news I wanted to hear, had to break the linking and now I am caught in the account linking failure message . But unlike you guyās no apple products in my house. Guess I am officially SOL now. 
looks like it might not just be HA stuff
just FYI I opened a new topic Google Assistant - Account Linking Failed
As this topic has so many postingsā¦
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This worked for meā¦
I use AutoVoice with Dialogflow to send Intents and return values to query devices. I use shortcuts so I donāt have to directly ask for AutoVoice. Itās a bit of a workaround but works great.