Do you have a cover or screen protector on the phone? If the proximity sensor is obstructed, this error can trigger. More information on the issue and the location of the sensor can be found here. https://zentalk.asus.com/en/discussion/9521/camera-cannot-flip-general-issue/p1 If you have already verified that the sensor is not obstructed, then clear the storage and cache in your camera. If no improvement, back up all of your data and factory reset the phone. And if that doesn't work, please contact your local service center for a warranty repair.
I am having the same issue, after restart the rear camera works fine but once I flip it for selfie mode, it hangs and doesn't work. Then the camera app just goes black and no other application can access the camera too. Even I started facing this issue on 2nd March after installing the update. The camera randomly stopped working. I never had to restart my phone due to camera problems, there were none, other then the usual cannot flip, it always worked after retracting. I took the SMMI test and it failed. Now today, I factory reset my phone and it still doesn't work. Have you been able to fix it in your Your phone Rupesh?