How to Mirror iPhone

Are you referring to the iPhone Mirroring app? If so make sure you are starting the session on your Mac with the iPhone locked and nearby. Further instructions are available here.

iPhone Mirroring: Use your iPhone from your Mac - Apple Support

“How to Mirror iPhone: Sometimes it works and sometime it doesn't. There are said to be instructions on the iPhone but I cannot find them? It is a horrible badly designed app. There, I've said it.“

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Report this to Apple:

Do your part: 

It’s great that you’ve said it here. But, there are just user-to-user forums. So, report this directly to Apple. Let Apple know what is going on here. Provide them with feedback on this. They may not get back to you directly, but the more feedback they received on this, the more they will know what is going on, and what needs to be fixed in updates to come.

1. Go Here: Feedback - iPhone - Apple
2. Comment: on when you came across this issue, and how you have troubleshot it this far.
3. Select: "Bug Report" for the "Feedback Type"
4. Proceed from there aw necessary

One Method:

Mirroring iPhone to Roku TV:

1. Try this App: Screen Mirroring for Roku - App Store
2. As it Reads: "Screen Mirroring for Roku is the easy mirror app to share your iPhone or iPad screen on your Roku Streaming Player"

“I don't have a Roku TV. I have an iPhone and a MacBook Air M4 so I don't want to be buying apps. to connect them. I have sent the Feedback to Apple”

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Thanks for doing your part. It will help many, for certain!

As for Roku:

Forget about an app; the Apple TV app and Apple Music apps are built in —it’s the dongle and remote, playing bar, or TV you’d purchase.