If your button is missing then you can first doing a cache cleaning, to do this please follow the path by holding the camera app button > app info > Storage and cache, in here you can clear the cache and the storage.
If this still gives issues then try to see if there is something wrong with the camera module.
For this please go to the calculator app and press ".12345+=" there you can select Single Test and you can try the front camera, if there is an issue with the result or there is no front camera selector then please reach out to your closest ASUS Service Center.
https://www.asus.com/support/service-center-location/
