"Attempt to index number with 'Best'"?

So - I'm working on a remake of Super Check Point, and i need bests that save. Here's what i have so far

local DataStoreService = game:GetService("DataStoreService")
local playerData = DataStoreService:GetDataStore("PlayerData")
local db = 0

local function UpdateStats(plr, stage, mode, dataa, datab)
    local data = playerData:GetAsync(plr.UserId)
    local best = plr:WaitForChild("Best")
    local prbest = plr:WaitForChild("PRBest")
    local plrdata = ("Player_" .. plr.UserId)
    if mode == "Standard Run" then
        if dataa < stage then
            best.Value = stage
            local success, err = pcall(function()
                playerData:SetAsync(plrdata, best.Value)
            end)
            if not success then
                warn('Could not save data!')
            end
        end
    else 
        if datab < stage then
            prbest.Value = stage
            local success, err = pcall(function()
                playerData:SetAsync(plrdata, prbest.Value)
            end)
            if not success then
                warn('Could not save data!')
            end
        end
    end
end


local function onPlayerJoin(plr)  -- Runs when players join
    local leaderstats = plr:WaitForChild("leaderstats")
    local best = Instance.new("NumberValue")
    best.Parent = plr
    best.Name = "Best"
    local prbest = Instance.new("NumberValue")
    prbest.Parent = plr
    prbest.Name = "PRBest"
    local playerUserId = "Player_" .. plr.UserId
    local data = playerData:GetAsync(playerUserId)
    print(playerUserId)
    if data then
        best.Value = data.Best --This is the line returning the error
        prbest.Value = data.PRBest
    else
        best.Value = 1
        prbest.Value = 1
    end
    plr.leaderstats.Stage.Changed:Connect(function()
        if db == 0 then
            db = 1
            UpdateStats(plr,plr.leaderstats.Stage.Value,plr.leaderstats.Mode.Value, best.Value, prbest.Value)
            wait(0.2)
            db = 0
        end
    end)
end

game.Players.PlayerAdded:Connect(onPlayerJoin)

but when I run it, it returns the error "Attempt to index number with 'Best'". Can I get some help here? I don't know why its doing this.