How to fix camera in iPhone with unknown camera message?

Did you have your camera replaced? Then it could be that it is replaced by a non original part. Didn't you have your camera replaced? Then there is a severe hardware error going on. Please Contact - Official Apple Support to set up hardware service

Get an appointment at an Apple Store for an evaluation of your phone.

I have the same exact issue. It randomly started a month ago. I have never had my camera replace or iPhone opened ever. This conveniently occurred almost exactly when my AppleCare expired. I did a remote scan and they told me it was a hardware issue. Now I have to find a time to go all the way to the Apple Store where appointments need to be booked weeks in advance for an issue that has no basis in anything I did. Just revert the update or send me a new phone. Apple still doing the 2 year curse.