UI Box to appear on mouse position

Im creating a UI box to appear as a "loading" bar before instantiating an object over it and destroying the GUI box. The way my game is set up now is the object that appears after the loading bar completes is instantiated on my mouse position in an OnMouse button down method. Everything works correctly except the UI Box is spawned in the upper left corner of my screen when I insatiate the game object. Ill post the code below to clarify.

My script attached to my object im instantiating:

 public class buildProgBar : MonoBehaviour
 {
     private buildItem[] gameObjects;
     float timeLeft;
     public float barDisplay;
     private Vector2 mousePos;
     private Vector2 size;
     public Texture2D emptyTex;
     public Texture2D fullTex;
     private SpriteRenderer sprite;
 
     private void Start()
     {
         gameObjects = GameObject.Find("BG").GetComponent<GameManager>().buildItems;
         timeLeft = gameObjects[GameManager.selectedStruct].BuildTime;
         sprite = gameObject.GetComponent<SpriteRenderer>();
         sprite.enabled = false;
         size = new Vector2(100, 15);
         mousePos = new Vector2(GameManager.spawnPosition.x, GameManager.spawnPosition.y);
 
     }
 
 
 
     void OnGUI()
     {
 
 
         //BG
         GUI.BeginGroup(new Rect(mousePos.x, mousePos.y, size.x, size.y));
         GUI.Box(new Rect(0, 0, size.x, size.y), fullTex);
 
         //Fill
         GUI.BeginGroup(new Rect(0, 0, size.x * barDisplay, size.y));
         GUI.Box(new Rect(0, 0, size.x, size.y), emptyTex);
         GUI.EndGroup();
         GUI.EndGroup();
     }
 
 
     private void Update()
     {
 
         if (timeLeft > 0)
         {
             timeLeft -= Time.deltaTime;
             barDisplay = timeLeft / gameObjects[GameManager.selectedStruct].BuildTime;
         }
         if (timeLeft <= 0)
         {
             sprite.enabled = true;
             enabled = false;
         }
     }

My script called by my Game Manager:

   private void OnMouseDown()
 {
     if (Input.GetMouseButtonDown(0) && !helpers.IsPointerOverUIObject())
     {

         spawnPosition = Camera.main.ScreenToWorldPoint(Input.mousePosition);
         spawnPosition.z = 0.0f;
         print(spawnPosition);
         selectedBI = buildItems[selectedStruct];
         toInstantiate = selectedBI.structure;
         if (buildStats.buildPoints >= selectedBI.cost && buildStats.maxPowerLevel + selectedBI.powerCost >= buildStats.maxPowerLevel)
         {
             buildStats.buildPoints -= selectedBI.cost;
             GameObject toBuild = Instantiate(toInstantiate, spawnPosition, Quaternion.identity);
             toBuild.transform.SetParent(mainCanvas.transform);
             buildStats.availablePower += selectedBI.powerCost;
             buildStats.maxPowerLevel += selectedBI.powerGain;
             
         }

     }
 }

Ive only been working in unity for around 3 days so pardon my mistakes. Thank you!