There are two things required while you mirror iPhone to Roku. They are setting up your Roku receiver and setting up the Roku app on your iPhone. You can also stream the iPhone to Roku using Photo+. If you have a problem with your iPhone controlling Roku, you can verify below Screen Mirror Devices for the possible blocked device. The blocked device will be displayed under Always Blocked Devices. The devices have to be added manually to the list.
The screen mirroring issue may occur due to various reasons. Check that the devices are connected properly. Check if the internet connection is fast. Try restarting your Roku device. If you still have screen mirror issue, reboot your iPhone once. These steps should resolve the mirroring issue. If the problem persists, reset the Roku device, re-configure it, and try screen mirroring.