Thanks for reaching out and I'm sorry to hear that you're having an issue connecting your Chromecast device to the network. Let's check this further.
A few questions: what type of Chromecast device do you have? What's the LED status on your Chromecast? Please manually connect to the Chromecast SSID. Here's how: go to the phone settings > Open Wi-Fi and connect to the Chromecast SSID (Example: Chromecast1234, Living Room TV, Bedroom TV, etc.) > Go to the Google Home app and try to set it up again.