I tried to integrate Nest into HA a year or two and after hours of frustration I eventually gave up. I tried again this morning and have had no better luck.
Following all the steps described here
Once done I am back in HA attempting to integrate. Once I enter my Device access ID I click “Submit” and a new web page opens. The first time the only thing on this page was “Opps, something went wrong”. I review my work and tried again. This time, after the submit, I get “Can’t link to Ha-Nest
Please contact Ha-Nest if the issue persists”
It may be that there’s authentication data lying around from last time. If it shows up in “Settings > Devices and Services > 3-dots > Application Credentials”, delete and retry.
Got a pleasant surprise this afternoon. I noticed that Google Nest was now on my Integration screen with the Configure button. Repeated the same steps as above and voila… It worked.