need help to fix GUI script ?

All you need to do is get rid of the new function MouseHover except for the item.ImageLabel.Image line, you're trying to call the function inside the function with your script.

In your script it was going like this.

function Msg()
print("Hi")
Msg()
end

--The script is only reading this part.

local gui = script.Parent
local slots = gui.Frame.ScrollingFrame.FrameH
local hats = slots:GetChildren()
local item = gui.Frame.item

function findfirstchild(slots)
	if slots.Name == "a1" then
		for i = 1,#hats do
			hats[i].MouseEnter:connect(function()
				item.ImageLabel.Image = "http://www.roblox.com/Thumbs/Asset.ashx?format=png&width=420&height=420&assetId="..hats[i].id.Value..""
				print("successful")
			end)
		end
	end
end