Open the BP_PlayerInventory then open the UnequipWeapon function and add the highlighted nodes shown in the image below.

Next copy the video below, you code in the UnequipWeapon function will look different as the video was made after I tieded the code up but that doesnt effect the steps in the video.

Next open the FreeSlotSearch collapsed node and copy the image below.

Next open the EquipWeaponFromSlot function and add the nodes shown in the images below, this has been split up into 2 images.

Next open the BP_WeaponSlot blueprint then open the OnDrop function and copy the image below.