Attempt to index nil with 'destroy' | Trying to destroy a GUI | (?)

I followed a youtube tutorial, and I know the very basics of scripting but I'm gradually learning. When attempting to run the last part of the script, I get the error "ServerScriptService.OverheadHandler:17:attempt to index nil with 'destroy'" Can anyone help?

local serverStorage = game:GetService("ServerStorage")

game.Players.PlayerAdded:Connect(function(plr)
	plr.CharacterAdded:Connect(function(char)
		local level = plr:WaitForChild("leaderstats").Rank.Level
		local levelVal = level.Value
		local rankTag = serverStorage:FindFirstChild(levelVal):Clone()
		rankTag.Parent = char.Head
		plr.leaderstats.Rank.Level.Changed:Connect(function()
			local level = plr:WaitForChild("leaderstats").Rank.Level
			local levelVal = level.Value
			local rankTag = serverStorage:FindFirstChild(levelVal):Clone()
			rankTag.Parent = char.Head
			local lastRank = levelVal - 1
			
			if lastRank > 0 then
				char.Head:FindFirstChild(lastRank):Destroy()
			end
		end)
	end)
end)