Please make sure your Chromecast is on the same network as the device you are viewing the video event or rental. If that is the case, and you still do not see the casting icon, open your Google Home App, select your Chromecast from the device list, click the gear icon (settings), then tap the three dots in the upper right-hand corner and select "Reboot."
Refresh the event, and you should be good to go!
Please note: iOS devices cannot Cast directly from the Chrome web browser. This is an Apple Restriction that cannot be bypassed.