To use Google Chromecast, you need the most recent version of the Prime Video app for either iOS or Android.
Note: Your iOS or Android device must be connected to the same Wi-Fi network as your Chromecast. Please ensure that your Chromecast, whether standalone or built-in to another device, is up to date. Also ensure that your Prime Video app and iOS or Android device are also up to date. If you are running an Android device, ensure that Google Play Services are also up to date.
Note: If you wish to change subtitle settings on Chromecast, first disconnect your Chromecast, then navigate to your iOS or Android device's accessibility settings to adjust the caption or subtitle settings there.