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)