I can't fix these issues and there so annoying can anyone help and if its simple I will feel so dumb
It’s searching for Inventory to be able to Add an Item, meaning the object that has Inventory.cs attached to. Null reference means that it’s not finding Manifestation of such object. The manifestation is the Instance that can for example be seen in the Hierarchy.
You have an error on line 41 of Inventory.cs. Show that file here, with code tags (the Code: button ^ in the edit ribbon)