Hi
@analema The message you are getting normally only appear when the proximity sensor is covered or dirty and then it shouldn't flip.
Here are some trouble shooting steps you can try:
1.I will recommend you to clear cache in camera app and system wide.
2.If this doesn't help then you could perform a factory reset. Keep in mind to back up your data before performing factory reset.
3.If after this tips your camera error remains then you should contact your nearest Asus Service Center to get your device check.