why does mny deaths leaderboard not appear?

Try making the data show up if it is nil, cuz right now it only shows up if it isn't nil.

local dss = game:GetService("DataStoreService")

local deathsDataStore = dss:GetDataStore("deathsDataStore")

game.Players.PlayerAdded:Connect(function(player)

    local leaderstats = player:WaitForChild("leaderstats")

    local deaths = Instance.new("IntValue")
    deaths.Name = "Deaths"
    deaths.Parent = leaderstats

    local data
    local success, errormsg = pcall(function()
        data = deathsDataStore:GetAsync(player.UserId.."-deaths")
    end)

    if data == nil then
          data = 0
    end

    if data ~= nil then
        deaths.Value = data
        print(player.Name.."'s deaths data was retrieved!")
    else
        print("Error while getting "..player.Name.."'s deaths data.")
        warn(errormsg)
    end

end)

game.Players.PlayerRemoving:Connect(function(player)

    local success, errormsg = pcall(function()
        deathsDataStore:SetAsync(player.UserId.."deaths",player.leaderstats.Deaths.Value)
    end)

    if success then
        print(player.Name.."'s deaths data was saved!")
    else
        print("Error when saving "..player.Name.."'s deaths data.")
        warn(errormsg)
    end

end)

Data being nil means the player has no record of their Deaths in the datastore.