Error when trying to access a part using string values?

So UpgradeWeapon is a stringvalue and this is my script in serverscriptservice. Why does it return this error? How can I access the part?

ServerScriptService.Upgrade Handler.Main:12: invalid argument #2 (string expected, got Instance)

local ReplicatedStorage = game:GetService("ReplicatedStorage")

local remoteEvent = ReplicatedStorage:WaitForChild("UpgradeEvent")


local function upgrade(player,UpgradeWeapon,UpgradeAmount)
	
	local Weapon = UpgradeWeapon
	
	print ("Upgrade Event Recieved")
	
	player.Inventory[Weapon].Damage.Value = player.Inventory[UpgradeWeapon].Damage.Value + UpgradeAmount
	
	


	end


remoteEvent.OnServerEvent:Connect(upgrade)

try changing

player.Inventory[Weapon].Damage.Value = player.Inventory[UpgradeWeapon].Damage.Value + UpgradeAmount

to

player.Inventory[Weapon].Damage.Value = player.Inventory[UpgradeWeapon].Damage.Value + UpgradeAmount.Value

the error is in line 12, and it is happening because you're giving an instance not a string

-- Pretty sure you are firing the actual tool instead of the name of the tool. try 

local Weapon = tostring(UpgradeWeapon)
player.Inventory[Weapon].Damage.Value += tonumber(UpgradeAmount)