How do I relate level and XP?

I might fixed your script

local levels = {
    ["level0"] = 0;
    ["level1"] = 500;
    ["level2"] = 1000;
    ["level3"] = 2000;
    ["level4"] = 3000;
    ["level5"] = 5000
}
local level = 0
script.Parent.XP.Changed:Connect(function(con)
        if con > levels["level"..level] then
            print(levels["level"..level])     
    end
end)

local players = game:GetService("Players")
local rs = game:GetService("ReplicatedStorage")

local loadedPlayers = {}
local dataStore = game:GetService("DataStoreService"):GetDataStore("Datastore")
local dataStoreWait = 150


-- CONFIGURATIONS
local containerName = "leaderstats" -- what the object containing the level and experience will be called
local levelName = "Level" -- what the level will be called
local experienceName = "XP" -- what the experience will be called

local saves = true -- if the stats will be saved in this script

local function equation(l)
        -- l is level
    --[[ Summation for small experience level-ups
        local sum = 0

        for i = 1,l do
            sum = sum + i
        end
    --]]

    return l * 100
end

-- FUNCTIONS

local function setUp(o)
    local c,l,x = o:WaitForChild(containerName,.1)

    if not c then
        c = Instance.new("Folder")

        c.Name = containerName
        c.Parent = o
    end

    l,x = c:WaitForChild(levelName,.1),c:WaitForChild(experienceName,.1)

    if not l then
        l = Instance.new("IntValue")

        l.Value = 1
        l.Name = levelName
        l.Parent = c
    end

    if not x then
        x = Instance.new("NumberValue")

        x.Name = experienceName
        x.Parent = c
    end

    return c,l,x
end


local function getLevel(l,x) -- calculates the level
        -- l is level and x is experience
    local endLevel = l
    local endExperience = x

    if x >= equation(l) then
        endLevel,endExperience = getLevel(l + 1, x - equation(l))
    end

    return endLevel,endExperience
end


local function attempt(f,t)
    local s,x
    t = t or 3

    for i = 1,t do
        s,x = pcall(f)

        if s then
            return x
        end
    end

    warn(x)
end


local function saveData(p,d)
        -- p is player and d is data (the container)
    local data = attempt(function() return dataStore:GetAsync(p.UserId) end)

    if data then
        attempt(function() dataStore:SetAsync(p.UserId,{L = d:WaitForChild(levelName).Value;X = d:WaitForChild(experienceName).Value}) end)
    else
        attempt(function() return dataStore:SetAsync(p.UserId,  {L = d:WaitForChild(levelName).Value;X = d:WaitForChild(experienceName).Value}  ) end)
    end
end


local function loadData(p,d)
        -- p is player and d is data (the container)
    local data = attempt(function() return dataStore:GetAsync(p.UserId) end)

    if data then
        d:WaitForChild(levelName).Value = data.L
        d:WaitForChild(experienceName).Value = data.X
    else
        attempt(function() return dataStore:SetAsync(p.UserId,  {L = d:WaitForChild(levelName).Value;X = d:WaitForChild(experienceName).Value}  ) end)
    end
end


local function calculateWait()
    local pCount = #players:GetPlayers()

    return math.max(15,300/(60 + pCount * 10) )
end




-- PLAYER STUFF

local function playerAdded(p)
    local container,level,experience = setUp(p)
    local experienceUpdateDebounce = true -- woah

    if saves then
        loadData(p,container)
        loadedPlayers[p] = container
    end

    experience.Changed:Connect(function()
        if experienceUpdateDebounce then
            experienceUpdateDebounce = false
            level.Value,experience.Value = getLevel(level.Value,experience.Value)

            wait(.1)
            experienceUpdateDebounce = true
        end
    end)

    dataStoreWait = calculateWait()
end


for i,v in pairs(players:GetChildren()) do
    spawn(function() playerAdded(v) end)
end

game.Players.PlayerAdded:Connect(playerAdded)

game.Players.PlayerRemoving:Connect(function(p)
    dataStoreWait = calculateWait()

    if loadedPlayers[p] then
        saveData(p,p:WaitForChild(containerName))
        loadedPlayers[p] = nil
    end
end)



if saves then
    local lastTick = tick()

    while true do
        wait(1)
        local ticked = tick() - lastTick

        if ticked >= dataStoreWait then
            lastTick = tick()

            for i,v in pairs(loadedPlayers) do
                saveData(i,v)
            end
        end
    end
end

if game:GetService("RunService"):IsStudio() then game:BindToClose(function() wait(3) end) end