First of all it's not an error, it's a warning.
Since your script is derived from MonoBehaviour your class already has a member called "camera". This member (inherited from Component) returns the Camera component that is attached to the same GameObject as your script. If there is no Camera it returns null.
You named your variable also "camera". That will hide the member that is already there. If you don't need the camera property of Component you can use the new keyword when declaring the variable: