coin

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Coin : MonoBehaviour 
{
    public GameObject coinPoof;
    private static int coinsCollected = 0;

    public int CoinsCollected
    {
        get { return coinsCollected; }
        set { coinsCollected = value; }
    }

    public void OnCoinClicked()
    {
        // Use gameObject.transform.position directly ( it is a Vector3! )
        Instantiate(coinPoof, gameObject.transform.position, Quaternion.Euler(-100f, 0, 0));
        CoinsCollected++;
        Destroy(gameObject);
    }
}

door

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Door : MonoBehaviour 
{
    public AudioClip[] soundFiles;
    public AudioSource soundSource;
    bool locked = true;
    bool opening = false;
    private float fullHeight = 5.0f;
    
    void Update() {
        if (opening && transform.position.y < fullHeight)
        {
            transform.Translate(0, 2.5f * Time.deltaTime, 0);
        }
    }

    public void OnDoorClicked() {
        if (!locked)
        {
            soundSource.clip = soundFiles[doorOpen];
            soundSource.Play();
            opening = true;
        }
        else
        {
            soundSource.clip = soundFiles[doorLocked];
            soundSource.Play();
        }
    }

    public void Unlock()
    {
        locked = false;
    }
}

    

key

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Key : MonoBehaviour 
{
    // Rather than GetComponent<Door>() below, just reference it directly
    public Door Door; 
    public GameObject Keypoof;

    void Update()
    {
    	//Not required, but for fun why not try adding a Key Floating Animation here :)
    }

    public void OnKeyClicked()
    {
    	// Use gameObject.transform.position directly ( it is a Vector3! )
        Instantiate(Keypoof, gameObject.transform.position, transform.rotation);
	
        // Call the Unlock() method on the Door
        Door.Unlock();
	
        // Destroy the key. Check the Unity documentation on how to use Destroy
        Destroy(gameObject);
    }
}