I took a very quick look at the utube channel, there’s a whole series on inventory, so… I haven’t watched it.
There’s a problem with your code in the first pic. You made a slot structure, but the item and quantity are both empty. So you’re looking for an empty slot structure. Unless you specifically make your array to contain ( say ) 20 empty slots, this search will always fail.
If you’re trying to add, for instance, the shotgun for this first time, then you would need to use the array ADD node.
Where did you get the loop concept from? Surely the idea is to just make this tutorial stuff work?