Instantiate not spawning at correct position

That code doesn't compile. Did you copy / paste that directly?

I set the parent / prefab as inspector assignments as I didn't have your Prefab$$anonymous$$anager and I never use anything with Find in its name. This works fine:

 using UnityEngine;
 
 public class QuickSlot : $$anonymous$$onoBehaviour
 {
     public Transform container;
     public Transform prefab;
 
     // Use this for initialization
     void Awake()
     {
         // Generate inventory and quick slots UI
         Vector3[] quickSlotPositions = new Vector3[] {
             new Vector3(-605, -605, 0),
             new Vector3(-500, -605, 0),
             new Vector3(-395, -605, 0),
             new Vector3(-290, -605, 0),
             new Vector3(-185, -605, 0),
             new Vector3(185, -605, 0),
             new Vector3(290, -605, 0),
             new Vector3(395, -605, 0),
             new Vector3(500, -605, 0),
             new Vector3(605, -605, 0)
         };
 
         for (int i = 0; i < quickSlotPositions.Length; i++)
         {
             print(quickSlotPositions[i]);
             var newQuickSlot = Instantiate(this.prefab, quickSlotPositions[i], Quaternion.identity, this.container);
         }
     }
 }