You cannot Chromecast directly to a Roku device
Roku does not support Google Cast natively
Use a Chromecast device plugged into your TV if you want Chromecast support
Use the Roku mobile app or supported casting apps instead
On Android, use screen mirroring if your phone and Roku support it
On Windows, use the Cast or Project feature if your Roku supports screen mirroring
On iPhone, use AirPlay if your Roku device supports AirPlay
Check the app you are using for a Roku Cast or Play on Roku option
Make sure both devices are on the same Wi-Fi network