Unable to reference a gameobject from a script.

I have two scenes Main and Game (both of them under a new empty containing the whole scene.
When i click play i want the gameobject (tagged "AllScene") to turn off so i cant see the MainScene.

Code (CSharp):

1.  
2. public class MenuScript : MonoBehaviour
3. {
4.     public void Play()
5.     {
6.         SceneManager.LoadScene("GameSceneLight",LoadSceneMode.Additive);
7.         MenuScene.SetActive(false);
8.     }
9.     }

When I click back from the game scene, I want the MainScene to turn back on, and the game scene (tagged "GameScene") to delete itself.

Code (CSharp):

1. public class GameOver : MonoBehaviour
2. {
3.     public GameObject GameScene;
4.     public GameObject MenuScene;
5.     void Start()
6.     {
7.         MenuScene = GameObject.FindGameObjectWithTag("AllScene");
8. }
9.     public void BackToMenuFromGame ()
10.     {
11.         GameScene.SetActive(false);
12.         MenuScene.SetActive(true);
13.     }
14. }

But the second part won't work.

Clearly, the No cameras rendering showing up is because the Main scene won't load, meaning no scenes, no cameras, are available.
Btw is my 4th day since started learning c# & unity. No need to say im stupid, I know

 

The scene is loaded - you can see it in the Hierarchy panel - but it's inactive, hence the No camera rendering message.

Clearly there's more to these scripts (given the error is on line 42 of a 14 line paste), and what's missing is important. Most importantly, how and when is GameScene being set?

 

Yeah there is more to it, but that code is useless. The error i get on line 42 is this error. I shortened everything to the core because thats the only problem i have.
When the game ends, GameOver is called, which has the role of finding the game object with the "AllScene" tag

Code (CSharp):

1.  
2. [LIST=1]
3. [*]    void Start()
4. [*]    {
5. [*]        MenuScene = GameObject.FindGameObjectWithTag("AllScene");
6. [*]     }
7. [/LIST]
8. ;

Which it does not, because when i call it,

Code (CSharp):

1.         public void BackToMenuFromGame ()
2.         {
3.             GameScene.SetActive(false);
4.             MenuScene.SetActive(true);
5.         }

I get the error at line 42.
The problem is, i just cant get the gameobject to be found during game by the tag.

 

That's where the error is, but that may not be where the problem is. In particular, I asked:

Which I still don't see anywhere.

Since you didn't post the full script I can't see your line numbers, this is absolutely meaningless to me.