Problem loading the main level scene from the main menu

I honestly don't know what is wrong, when I load the main game scene from the main menu scene, it is very broken, shaders don't work, and one of the scripts seems to not work either.

The script is supposed to spawn sections of the level, but doesn't do anything.

The game scene works fine when I enter play mode on it.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.SceneManagement;
 
 public class MapGenerator : MonoBehaviour{
     // Start is called before the first frame update
 
     public GameObject fpiece;
     public GameObject apiece;
     public GameObject spiece;
     public Vector3 spos;
     public float rate;
     public int dirRate;
     public Quaternion dir;
     public int i;
     public float speed;
     public int spawnTimer;
     public int spawnTimer2;
     public float timer;
     public GameObject rightCon;
     public GameObject leftCon;
     public float totalVelocity;
 
     void Start(){
         StartCoroutine("generator");
         StartCoroutine("genObst");
         StartCoroutine("genObst2");
     }
 
     void Update(){
         rate = 1;
         timer = 30;
         totalVelocity = Mathf.Abs(rightCon.GetComponent<ControllerVelocity>().velocity.magnitude) + Mathf.Abs(leftCon.GetComponent<ControllerVelocity>().velocity.magnitude);
         speed = (0-totalVelocity) / 8;
         rate /= (0-speed) * 15;
         timer /= (0-speed) * 500;
         if(timer > 30){
             timer = 30;
         }
         if(rate > 1){
             rate = 1;
         }
     }
 
     IEnumerator generator (){
         print("what");
         while(true){
             if(totalVelocity < -0.01f){
             for(i =0; i < dirRate; i++){
             Instantiate(fpiece, spos, transform.rotation);
             spawnTimer ++;
             spawnTimer2 ++;
             yield return new WaitForSeconds(rate);
             }
             if(Random.Range(0,2) == 0){
                 for(int c = Random.Range(4, 30); c > 0; c--){
                     transform.Rotate(0, Random.Range(1, 2), 0);
                     Instantiate(fpiece, spos, transform.rotation);
                     spawnTimer ++;
                     spawnTimer2 ++;
                     yield return new WaitForSeconds(rate);
                 }
             }else if(Random.Range(0,2) == 1){
                 for(int c = Random.Range(4, 30); c > 0; c--){
                     transform.Rotate(0, Random.Range(-1, -2), 0);
                     Instantiate(fpiece, spos, transform.rotation);
                     spawnTimer ++;
                     spawnTimer2 ++;
                     yield return new WaitForSeconds(rate);
                 }
             }
             yield return new WaitForSeconds(0.01f);
             i = 0;
             }
             yield return new WaitForSeconds(0.01f);
     }
 }
 
     IEnumerator genObst (){
         while(true){
             int t = Random.Range(1, 10);
             while(spawnTimer < t){
                 yield return new WaitForSeconds(0.01f);
             }
             Instantiate(apiece, spos, transform.rotation);
             spawnTimer = 0;
         }
     }
     IEnumerator genObst2 (){
         while(true){
             int t = Random.Range(1, 10);
             while(spawnTimer2 < t){
                 yield return new WaitForSeconds(0.01f);
             }
             Instantiate(spiece, spos, transform.rotation);
             spawnTimer2 = 0;
         }
     }
 
 }