This seems to be an on-going problem with Google Nest devices. If you search this forum for "assisting device," you'll find several other post threads, each with varying solutions and varying results. I'm just another Nest customer and haven't encountered the problem myself.
I have my own theory, however, one that I haven't seen posted anywhere. I think, when you already have Google Nest devices installed and try to install a new one (with your mobile device running the Google Home app close to the new device), the new Google Nest device tries to communicate with an existing Google Nest device via bluetooth, using some sort of hierarchy, in order to obtain the password for your WiFi network. If my theory is right, then you may need to configure your new Google Nest device in close proximity to an existing working Google Nest device and get the new device configured and working before then moving it to its final location.
The only thing I base this guess on is that I encountered problems recently trying to get a new Google Nest Hub configured. After several attempts where it got to the last stage and then just wouldn't connect, it finally worked when it was located temporarily near an existing Google Nest Hub.
I'm sorry that didn't work. We are all just customers trying to figure out how to address this.
In this thread, the suggestion is to remove the first camera and then try to add the second, and then add the first one back. You can check out the thread. Or, as I mentioned above, search this forum for "assisting device" or "assisting Nest device" in quotes to see all of the other things customers have tried.
Google is not providing help on resolving this. Some customers think it's due to cameras that are shipped with older versions of their software. I don't know.