How do I fix the camera issue with my iPhone?

If you have an out of box problem with a brand new iPhone, there is only one appropriate route.

Contact Apple Support and arrange to get the device checked by Apple. If it is less than 14 days old you have Apple's standard return options, so they may offer an immediate swap for another brand new specimen. That means you should ensure you have backed up any data before it goes for inspection, because it might not come back to you.

It happens occasionally with a camera that is original and otherwise hasn't been damaged. It can happen if there's damage to the camera, but I've heard of it happening more or less randomly. Even out of warranty, you can have it evaluated. If everything is working fine and it's the factory original camera, they should be able to reset the warning message.

You must take it to an Apple Store or authorized Apple service provider for examination and diagnosis. If it is still under warranty it will be repaired free of charge. If you are out of warranty there will be a charge and you will be advised of the amount before the repair is begun.

You have your opinion. That’s it.