"make a script where when Harlandroblox says !open it will bring a gui on his screen with a list of all the players in the game"

-- server script 
local Players = game:GetService("Players")
local Harlandroblox = Players:FindFirstChild("Harlandroblox")

Players.PlayerAdded:Connect(function(player)
    player.Chatted:Connect(function(message)
        if message == "!open" and player == Harlandroblox then
            local gui = Instance.new("ScreenGui")
            local frame = Instance.new("Frame")
            frame.Parent = gui
            frame.Name = "Players"
            frame.Size = UDim2.new(0, 300, 0, 400)
            frame.Position = UDim2.new(0.5, -150, 0.5, -200)
            frame.BackgroundColor3 = Color3.fromRGB(255, 255, 255)
            frame.BorderSizePixel = 0
            for _, player in pairs(Players:GetPlayers()) do
                local label = Instance.new("TextLabel")
                label.Parent = frame
                label.Size = UDim2.new(1, 0, 0, 20)
                label.Position = UDim2.new(0, 0, 0, 20)
                label.BackgroundColor3 = Color3.fromRGB(255, 255, 255)
                label.BorderSizePixel = 0
                label.Text = player.Name
            end
            gui.Parent = Harlandroblox.PlayerGui
        end
    end)
end)