I can't save a class with scriptable objects into a class

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Hey guys, I’ve have totally no idea how to save a class with List of scriptableObjects

and here is the class that is being saved:

[System.Serializable]
public class Inventory
{
   public float totalCash;
   public float stoveUpgradeValue;
   public float cashGenUpgradeValue;
   public List<CounterData> counterDataList = new List<CounterData>();
   public List<PurchasedItems> purchasedItemsList = new List<PurchasedItems>();
   public List<PurchasedItems> ownedCountersList = new List<PurchasedItems>();
   public List<PurchasedItems> ownedRecipeList = new List<PurchasedItems>();
}

and here is the PurchasedItems class:

public string itemName;
public CounterSO counterSo;
public RecipeSO recipeSO;
public int amount;

and here is how i save

public void SaveToJson()
{ 
    string inventoryData = JsonUtility.ToJson(inventory);
    string filePath = Application.persistentDataPath + "/InventoryData.json";  
    inventory.totalCash += cashInPocket;
    inventory.stoveUpgradeValue += upgradeManager.stoveCurrentLevel;
    inventory.cashGenUpgradeValue += upgradeManager.cashGenCurrentLevel;
    Debug.Log(filePath);
    System.IO.File.WriteAllText(filePath, inventoryData);
}

public  void LoadFromJson()
{
    string filePath = Application.persistentDataPath + "/InventoryData.json";
    string inventoryData = System.IO.File.ReadAllText(filePath);
    inventory = JsonUtility.FromJson<Inventory>(inventoryData);
    CashManager.Instance.currentCash = inventory.totalCash;
    upgradeManager.stoveCurrentLevel = inventory.stoveUpgradeValue;
    upgradeManager.cashGenCurrentLevel = inventory.cashGenUpgradeValue;
}

the target here is to save the Inventory class that has List of PurchasedItems class which contains scriptable objects but the problem every day i open unity the scriptable objects reference becomes type mismatch

I have no idea why.