Restart the LG G4, this could fix camera failed problem. Hold the “Power” button and the “Home” button at the same time for 7 seconds until the phone turns off and vibrates. Go to Settings, open the Application manager and then go to the Camera app. Select on Force stop, clear data and clear cache. For more assistant support please check [https://guideusermanual.com/product-name-lg-g3-manual&po=134051&lang=English|LG G3 Manual.]