Go to the BP_MasterHoldable then to the BeginPlay collapsed node and select the NewOwner event and add a new input called Owner and set it to Actor. Then add another input called CurrentSocket and set it to Name. Then compile. Then copy the image below adding the highlighted nodes.
Next go to the BP_PlayerInventory and replace your current code in the area shown in the screenshots below with the nodes shown in the images. Location at the tops of the screenshots.
NOTE – in 4.25 and 4.26+ engine versions the AttachToComponent node is now called AttachActorToComponent.