public void Save()
    {
        saveGame.playerPosition = transform.position;
        BinaryFormatter bf = new BinaryFormatter();
        FileStream file = File.Create(Application.persistentDataPath + "/gamesave.dat");
        bf.Serialize(file, saveGame);
        file.Close();
    }

SerializationException: Type 'UnityEngine.MonoBehaviour' in Assembly 'UnityEngine.CoreModule, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' is not marked as serializable.