Hi there,
I've just cloned your repo and loaded the project in Unity 5.5.1p1. After loading, I've got this error in my console:
Assets/Scripts/BaseClasses/Item.cs(78,83): error CS0039: Cannot convert type 'Item' to 'UnityEngine.GameObject' via a built-in conversion
and here is the problematic line in Item.cs:

The problem is that the instantiated object is an Item which does not derive from a GameObject (but from a MonoBehaviour instead). One potential fix is to use the GameObject property of the instantiated Item:

Thanks,
Nika