iPhone Mirroring Error: 'iPhone is in use'

I had this problem trying to use it after I'd already used it earlier. To cut to the chase, this is the protocol to use mirroring:

1) open the mirroring app on mac and try to connect.

2) the app will ask to unlock the phone

3) the app will say that the iPhone is in use and will connect when it is NOT in use. The phone is "in use" WHILE you have it unlocked

4) lock the phone again, and the app will now connect and you are able to control all the phone apps from the mirror app on the mac

Note that once you initiate mirroring, so long as the phone remains Unlocked and nearby, you can quite the mirror app on mac and later restart it, and it will immediately resume mirroring. Presumably, if you unlock the phone, or restart the mac, mirroring will end and you'll need to restart it as above. Possibly, if you take the phone at a distance away, or maybe after some lengthy period of inactivity on the app, it will also disconnect mirroring (I didn't wait a long time to see...). But the app does appear to keep mirroring active after initially connecting, for awhile at least, without a need to repeat the connection protocol.

This actually makes sense - Apple would not want the user to try to control apps simultaneously from BOTH the phone and the mirroring app, only ONE should be in control at any one time. Unfortunately, locking the phone is not intuitive to make the iPhone NOT "in use", but it is a simple fix to the problem.

The iPhone needs to be locked. Make sure you are following all of the instructions here, iPhone Mirroring: Use your iPhone from your Mac - Apple Support

Do not touch the screen or swipe up or open any app. Thats' it

One error in my summary above:

Note that once you initiate mirroring, so long as the phone remains locked and nearby, you can quit the mirror app on mac and later restart it, and it will immediately resume mirroring.