In the official video: “Tutorial - Action RPG Inventory System - Merging Into Your Project” at the time of this link, for one example: https://youtu.be/jXYA02qQo5Y?t=1551
There is a “SET” box in the BP “Container_Slot” that is connected immediately after the “Inventory Manager Component” … also shown in the attached image taken from the same video.
That same “SET” box also existed in the same video for at least the Inventory Layout BP and maybe one more Inventory BP. However, when using the current version of the ARPG Inventory System, those “SET” boxes do not exist, so it is not clear as to what to connect the “Inventory Manager Component” into. As it is, the inventory system does not work without a connection and I have not seen any post, specific to Multiplayer TopDown Kit from the same creator, that addresses this situation.