My game object does not show in the game as I have designed it on gui (Unity and C#)

I am learning Unity and coding a lot with it lately. Following a toutorial I found on youtube, I made an inventory system so that I can randomly create items and hold them. I also want the player to be able to see an item description as well as the item name using a tooltip.

Following said toutorial, I have made the tooltip but when I play the game the UI design I have created does not carry over to the tooltip. It only shows me the text, where there should be a box.
How this looks is shown in the following screenshot:

There red square is where the window should appear like in the prefab that is created in the lower left of the screen.

I have been using the following code. The itemInfo that is set on the itemText prefab:

namespace Code.Inventory
{
    public class ItemInfo : MonoBehaviour
    {
        public Text _itemName;
        public Text _itemDescription;

        public void SetUp(string name, string description)
        {
            _itemName.text = name;
            _itemDescription.text = description;    
        }
    }
}

The following code represents the item where I write a description for an item:

public class Item : ScriptableObject
    {
        public string _name = "Default Item";
        public Sprite _icon = null;
        public string _itemDescription = "Used to progress the story";
    
        /// <summary>
        /// Virtual means I can overwrite this function in another class once instanciated.
        /// This function is called when the item is actually clicked on. Momentarely its only logging stuff.
        /// </summary>
        public virtual void Use()
        {
            Debug.Log("Hello World" + _name);
        }
    
        /// <summary>
        /// Simply returns the item description that is doing to be created on the item class itself.
        /// </summary>
        /// <returns></returns>
        public virtual string GetItemDescription()
        {
            return _itemDescription;
        }
    }

And the following is where I use the GameManager to create and destroy the GameObject:

/// <summary>
        /// Displays item information when the mouse is hovered over an item.
        /// </summary>
        /// <param name="itemName"></param>
        /// <param name="itemDescription"></param>
        /// <param name="buttonPos"></param>
        public void DisplayItemInfo(string itemName, string itemDescription, Vector2 buttonPos)
        {
            if (_currentItemInfo != null)
            {
                Destroy(_currentItemInfo.gameObject);
            }
        
            // Create variables so that the tooltip window is moved to a different direction and not simply closed
            buttonPos.x -= 80;  
            buttonPos.y += 40;
            
            _currentItemInfo = Instantiate(_itemInfoPrefab, buttonPos, Quaternion.identity, _inventoryPanel);
            _currentItemInfo.GetComponent<ItemInfo>().SetUp(itemName, itemDescription);
        }
        
        /// <summary>
        /// Destroy Item info so it's not staying behind when the mouse is leaving.
        /// </summary>
        public void DestroyItemInfo()
        {
            if (_currentItemInfo != null)
            {
                float delayInSeconds = 0.5f; // Set the delay in seconds
        
                // Delay the destruction of the _currentItemInfo game object
                StartCoroutine(DelayedDestroy(_currentItemInfo.gameObject, delayInSeconds));
        
                _currentItemInfo = null; // Clear the _currentItemInfo variable
            }
        }

        IEnumerator DelayedDestroy(GameObject gameObjectToDestroy, float delayInSeconds)
        {
            // Wait for the specified delay
            yield return new WaitForSeconds(delayInSeconds);
            
            // Destroy the game object
            Destroy(gameObjectToDestroy);
        }

Last but not least, here I call said functions to actually do the work:

/// <summary>
        /// Call item description when moused over an item.
        /// </summary>
        public void OnCourserEnter()
        {
            // Call the data from the GameManager script
            GameManager.instance.DisplayItemInfo(_item.name, _item.GetItemDescription(), transform.position);
        }
        
        /// <summary>
        /// Close item description when mouse leave the item.
        /// </summary>
        public void OnCourserExit()
        {
            GameManager.instance.DestroyItemInfo();
        }

I was able to reproduce the red cross in 2 different ways.

The first is with the RectTransform's anchor presets in "normal" mode, and the second when the anchor presets are in Stretch mode.

In the first case the red cross happens when either the Width or Height (or both) are set as negative numbers in the Rect Transform, as shown in the pictures below.

With positive Width:

With negative width:

The second case is a bit more complicated to explain, but the concept is the same,and it happens when the Left, Top, Right or Bottom fields, make the object overlap with itself, as shown in the gif below to better understand it.

(Same concept applies to the other fields)

The only things that may cause this to happen that I can think of are:

1. The prefab is set up wrong
2. Something goes wrong when you Instantiate the object
3. Somewhere in the code you edit the Width or Height / Top, Bottom, Left or Right fields
4. There are problems with the Rect Transform of a parent of the Instantiated object (maybe _inventoryPanel?)

Hope this helps to solve the problem