This appears to be a recurring issue with the AirPlay2 support, however:
1) Verify you have the latest AirPlay support: Settings/System/System update/Check now - try again
2) Disable & Re-enable WiFi on the Apple devices - try again
3) Restart all devices (Rokus, Apples, router/modem) - try again
4) Reset the paired devices: Settings/Apple Airplay and HomeKit/Require Code/Reset Paired Devices - try again
5) Setup HomeKit: Settings/Apple Airplay and HomeKit/Setup HomeKit - try again