I am getting an error how can I fix it,

You are trying to create a MonoBehaviour using the ‘new’ keyword. This is not allowed. MonoBehaviours can only be added using AddComponent(). Alternatively, your script can inherit from ScriptableObject or no base class at allUnityEngine.MonoBehaviour:.ctor()

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

public class Item : MonoBehaviour
{
    public string itemismi,itembilgi;
    public int itemid, itemmiktar, itemdepomiktar, itemhasar;
    public Sprite itemicon;
    public GameObject itemmodel;
    public ItemType itemtipi;
    
    public enum ItemType
    {
        Silah,
        Malzeme,
        Yiyecek,
        Bos
    }

    public Item (string isim, string bilgi, int id, int miktar, int depo, int hasar, ItemType tip)
    {
        itemismi = isim;
        itembilgi = bilgi;
        itemid = id;
        itemmiktar = miktar;
        itemdepomiktar = depo;
        itemhasar = hasar;
        itemtipi = tip;

        itemicon = Resources.Load<Sprite> (id.ToString()); 
        itemmodel = Resources.Load<GameObject> ("Kup"); 
    } 
}

The issue you have is that you try to use your class Item as if it was a normal C# class. But this is not the case.

public class Item : MonoBehaviour //by this it is now a unity component…

By inheriting from MonoBehaviour you make it a unity component which can not stand alone when created. It must always belong to a GameObject. Thus using new in this context as well as constructors is not allowed.

Thus this constructor must be changed:

  public Item (string isim, string bilgi, int id, int miktar, int depo, int hasar, ItemType tip)
 {

to something like an initialize function

  public initialize(string isim, string bilgi, int id, int miktar, int depo, int hasar, ItemType tip)

Then you can do this:

  GameObject go = new GameObject(); //create a new GameObject in the scene.
  var item = go.AddComponent<Item>(); //add item component
  item.initialize(arg,arg,arg.....);   //call "constructor" replacement -> initialize function