"create a loot system system that shows a loot gui when players receive the item after killing a NPC"

-- server script 
local NPC = game.Workspace.NPC
local lootTable = {"Item1", "Item2", "Item3"}

local function onNPCKilled()
    local randomLoot = lootTable[math.random(#lootTable)]
    local players = game:GetService("Players"):GetPlayers()
    for i, player in ipairs(players) do
        local lootGui = player.PlayerGui:FindFirstChild("LootGui")
        if lootGui then
            lootGui.LootText.Text = "You received: " .. randomLoot
            lootGui.Visible = true
            wait(5)
            lootGui.Visible = false
        end
    end
end

NPC.Humanoid.Died:Connect(onNPCKilled)

-- local script in StarterGui 
local ScreenGui = Instance.new("ScreenGui")
ScreenGui.Name = "LootGui"
ScreenGui.Parent = game.Players.LocalPlayer.PlayerGui

local LootText = Instance.new("TextLabel")
LootText.Name = "LootText"
LootText.Parent = ScreenGui
LootText.Size = UDim2.new(0.3, 0, 0.1, 0)
LootText.Position = UDim2.new(0.35, 0, 0.4, 0)
LootText.BackgroundTransparency = 1
LootText.TextColor3 = Color3.fromRGB(255, 255, 255)
LootText.TextScaled = true
LootText.Text = ""
LootText.Visible = false