Problem solved. “ Manual setup (if you don’t have Home Assistant Cloud)” Google Assistant without Nabu Cloud works.
My problem was Duckdns related to my internet provider.
The ip4 address is just a nat address. This causes Google to try to access my HA via the ip4 address. I could try my luck, but then there was no response from Google.
The solution was incredibly simple. The ip4 on Duckdns was deleted and an automatic update was prevented.
It’s been running for 2 days without any problems. But now I have 2 new ones  My VPN WireGuard® is causing problems. The IP4 is gone and will probably have to display it again for the Fritzbox. And on Google I get:
Add capabilities
Seamless Setup Support local query
not selected.
But I’ll find that too. It doesn’t work, it doesn’t exist.
I get my HA login page, and there when I enter credentials I am getting message could not reach [test] homeasssistant. Please try again.
I even switched off rule in cloud flare, now I am getting following message
diagnosis update 2 : Solution
issue with ip and dns resolution
yes duck dns , no ip like other services can help you to get access over internet but if you using ipv6 only then your google home app show error "could not reach [test] all
solution you need ipv4 as well to resolve ipv4 if you don’t have ipv4 like me here is solution for that as well
buy domain or if you have domain configure with cloud flare it will resolve your ipv4 queries and transfer to you on you over tunnel
before you setup cloud flare make sure you turned of reverse proxy like nginx and duck dns or no ip or what ever you are using
setup Cloudflare in home assistant
then google home and Alexa will work without any error
I have the same issue, I filled out privacy policy , logos etc. I can not get logged in from my phone, tried iOS android, reinstalling google home app. Ridiculous how easy to setup Siri and home bridge but all these hoops just to get my devices into google home for my android devices.
Head to https://console.cloud.google.com/ select the correct project, from the navigation menu select APIs & Services and head onto the OAuth consent screen
If the publishing status is like in the image, click on BACK TO TESTING
After that, try to add the device again in the Google Home app and it should appear
I noticed that after cloudflare update my devices starts randomly going offline sometimes then I unlinked Hass from Google home and then I can´t likked it again. So I went under Cloudflare security and noticed under Events that Cloutflare are blocking Google with Bot fighting mode. After turning off Bot Fighting Mode I get linked my Hass project whitout problem.