Hey there guilhermelv,
I apologize for the late response, thank you for being patient! Were you able to fix that assistant issue? If not, I'd be happy to help out, as troubleshooting the Assistant can be tricky. First, I would recommend that you follow the steps listed in this article to make sure that everything is set up properly. For the Chromecast and Assistant to work together properly, make sure that your TV's HDMI-CEC settings are enabled, and even try changing the HDMI port that the Chromecast is plugged into as well.
I look forward to your response, and if you have any other questions or concerns for me, please let me know!