Instantiating and destroying unity prefabs

I am trying to create a platformer in unity and have a platform that randomly generates. I am able to get the platform object to spawn but cannot get it to be destroyed.

Any help on creating / destroying prefabs would be highly appreciated.

public GameObject platform;
private Vector2 screenBounds;

void Start()
{    // Spawn the platforms
    Vector2 spawnPosition = new Vector2();
    for ( int i=0; i < platformCount; i++) 
    {
        spawnPosition.y = Random.Range(1f , 10f);
        spawnPosition.x = Random.Range(-2.0f, 2.0f);
        instance = Instantiate(platform, spawnPosition, Quaternion.identity);
    }
    screenBounds = Camera.main.ScreenToWorldPoint(new Vector3(Screen.width, Screen.height, Camera.main.transform.position.z));
}

void FixedUpdate()
{
    if (instance != null) 
    {
        if (platform.transform.position.y < screenBounds.y * 5) 
        {
            Destroy(instance);
        }
    }
}

The main issue was already mentioned in the comments:

instance = Instantiate(platform, spawnPosition, Quaternion.identity);

this is re-assigned in a loop so it will only end up with the very last iteration's instance.

Further in your check you are using the original prefab

if (platform.transform.position.y < screenBounds.y * 5)

this is the prefab from the assets you instantiate so of course it never moves in the scene at all.

I would either split this into its very own script and do e.g.

public class Spawner : MonoBehaviour
{
    [SerialzieField] private OffScreenDestroy platformPrefab;

    void Start()
    {  
        for ( int i=0; i < platformCount; i++) 
        {
            var spawnPosition = new Vector2(Random.Range(1f , 10f), Random.Range(-2.0f, 2.0f));
            Instantiate(platformPrefab, spawnPosition, Quaternion.identity);
        }  
    }
}

and

// Attach this to your platform prefab
public class OffScreenDestroy : MonoBehaviour
{
    private static Vector2? screenBounds;

    private void FixedUpdate()
    {
        if(screenBounds == null) screenBounds = Camera.main.ScreenToWorldPoint(new Vector3(Screen.width, Screen.height, Camera.main.transform.position.z));

        if(transform.position.y < screenBounds.Value.y * 5) 
        {
            Destroy(gameObject);
        }
    }
}

Or alternatively keep them in a collection and do e.g.

public class Spawner : MonoBehaviour
{
    [SerializeField] private GameObject platformPrefab;

    private readonly List<GameObject> instances = new();
    private Vector2 screenBounds;

    private void Start()
    {  
        screenBounds = Camera.main.ScreenToWorldPoint(new Vector3(Screen.width, Screen.height, Camera.main.transform.position.z));

        for ( int i=0; i < platformCount; i++) 
        {
            var spawnPosition = new Vector2(Random.Range(1f , 10f), Random.Range(-2.0f, 2.0f));
            instances.Add(Instantiate(platformPrefab, spawnPosition, Quaternion.identity));
        }  
    }

    private void FixedUpdate()
    {
        for(var i = instances.Count -1; i >= 0; i--)
        {
            var instance = instances[i];
            if(instance.tranform.position.y < screenBounds.Value.y * 5) 
            {
                Destroy(instance);
            }
        }
    }
}