Look at the origin of the key, make sure the distance from the origin to the centre of the key is the same as the distance from the origin to the centre of the inventory slot object.
If you need help with that then screen shot the points of the inventory slot object and the keys.
The origin of the slot is near the bottom left, while the origin of the key is the top right. If you position each at the same point, they’ll be misaligned because of those mismatching origins.
I’d recommend you move the origin and centres of both the inventory slots and keys to the middle of their images. It’ll mean a slight repositioning of the slots in the UI, but it’ll make life a lot simpler.
There isn’t really much to show. When you create the keys, either placing them in the scene in the editor or via events, make sure you set the layer to Base Layer (or “”).
And when you collect the key and move it to the inventory slot, just add an extra action to change the key’s layer to UI (and change it’s Z-Order to ensure it appears above the inventory slot: