Perform troubleshooting steps 1-8 listed in this answer.
If the issue persists, then you may need to check that your router is configured to properly allow Google Cast traffic. Most home networks don’t require any special configuration, but sometimes you may need to change a setting or two on your router to allow streaming on your network. The link below lists compatible routers as well as the most commonly needed configuration changes on many popular home routers. Cast router compatibility.