This may be proprietary code in the Asus MB Programming. If so, you might have a much easier time of choosing a different keyboard shortcut.
You might follow the same steps here:
except checking for anything related to the camera / screenshot function or use of the fn keys you are trying to use.
If you do not find any that are being actively used - then you might try using dconf-editor to set the shortcut.
That the fn+F11 worked suggests that the function key is working properly and that it is the combination you are using that may be the trouble.