@Nestmess
You could try contacting Support (see below). This "cannot connect to joining device" error message has been reported in this forum for years now; it seems that, when you already have Google Nest devices, Google Nest's install process tries to contact one of those devices in order to obtain your Wi-Fi credentials from that device (rather than just letting you enter your Wi-Fi username and password), but that attempt to contact the "joining device or assisting device" fails for some reason, and Google Nest has apparently still not fixed the reason why it fails even though the problem has been occurring for 3 years now.
Some customers have had success turning off all their other Google Nest devices temporarily, and then attempting the reinstall again.
To contact Support:
1. Go to https://support.google.com/googlenest/gethelp.
2. Select a category/product family, and type in a brief description of the issue, then click "Next".
3. Click "Other", then click "Next step".
4. Under "Resources", just click "Next step". (Clicking one of the listed links will bring up a Help page.)
5. Under "Contact options", choose to get a phone call or chat with the team, depending on availability in your country.
