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Camera screen is black and after a few seconds this message is displayed: "Can't connect to the camera. Please make sure to close other apps that may use camera or flashlight".
All other phone functions are normal. I have tried several methods to recover, including factory reset.

I need help, please.

It looks like an hardware fault.
Maybe the camera module got disconnected from the motherboard.

If your device is still under warranty, i suggest to contact your seller to have it replaced.
If your warranty is due and if you are handy, you can open the phone and check such connection.

or make a factory reset...