As the error mention the equipping system can't find the prefab to spawn for the item you are trying to equipp.
From the logs I can assume you are using the integration with UCC.
To equip an Item you must define the prefab to spawn within an attribute in the ItemDefinition Editor.
It can either be a GameObject array
or a GameObject
The name must match exactly what you have in the Character Inventory Bridge
And of course the Item prefab must be setup like shown in the video tutorials, such that the UCC system can use that item.
I hope that helps, make sure to double check the documentation and the video tutorial as all of this is mentioned there and you might have missed other important things: