How would i transfer client code to server sided code?

I'm making this inventory toolbar and it works fine on the clients side but when it comes to making it work on the server side i have a bit of a problem getting the server sided code work with the clients code.

So i'm trying to use the _G function to transfer the clients code to the server but i'm not the best at it so i don't even know if that is possible to transfer the code to make it work with the FE.

I basically want some of the code to be on the client side cause it has to and some of the code to be on the server side just so it works with FE.

Client Script:

local gui = script.Parent:WaitForChild("ScreenGui")
	local Toolbar = gui.Toolbar
	
 _G.inv = {nil, nil, nil, nil, nil, nil, nil, nil, nil, nil}

local nextSlot = 1
local currentSlot = 1

function ToolHotbar(slot, empty, item)
	if empty then 
		local imageID = items[item]:FindFirstChild("imageID").Value
		Toolbar["Slot"..nextSlot].itemImage.Image = "rbxassetid://"..imageID
		Toolbar["Slot"..nextSlot].itemImage.Visible = true
		inv[nextSlot] = item
	end
end	

local function checkIfEmpty(empty, item)
	if inv[nextSlot] ~= nil then 
		nextSlot = nextSlot + 1
		currentSlot = currentSlot + 1
		if inv[nextSlot] ~= nil then
			nextSlot = nextSlot + 1
		end
		empty = true
		if nextSlot <= 10 then
			ToolHotbar(nextSlot, empty, item)
		else
			print("Your inventory is full please empty it.")
		end
	else
		print("Slot"..nextSlot.. " Is empty, Adding item...")
		empty = true
		ToolHotbar(nextSlot, empty, item)
	end
end

function addItem(empty, item)
	checkIfEmpty(empty, item)
end

Server Script:

_G.inv{nil, nil, nil, nil, nil, nil, nil, nil, nil, nil}
print(_G.inv)

Errors:

19:14:53.072 - ServerScriptService.ServerPlacement:6: attempt to call field 'inv' (a nil value)
19:14:53.072 - Stack Begin
19:14:53.073 - Script 'ServerScriptService.ServerPlacement', Line 6
19:14:53.073 - Stack End