The referenced script on this Behaviour is missing!

I'm trying to make a survival game --->https://www.youtube.com/watch?v=FKT7jIW6ORU<--- but the thing I try to kill can't die and the errors that I get say The referenced script on this Behaviour is missing!UnityEditorInternal.InternalEditorUtility:AddScriptComponentUnchecked(GameObject, MonoScript) UnityEditor.HostView:OnGUI()

and

Assets/Editor Utilities/Editor/MissingScriptResolver.cs(235,38): warning CS0618: UnityEditor.Undo.RegisterSceneUndo(string)' is obsolete: Use DestroyObjectImmediate, RegisterCreatedObjectUndo or RegisterUndo instead.'