Quest System Not Functioning at all no errors at all ?

Your current attempt does not work because you are trying to access the player's GUI through the StarterGui. You need access it through PlayerGui, which is parented under the Player instance.

In your Touched event, you need to check if the part touching belongs to the player's character. We can use the Players:GetPlayerFromCharacter() function to get the player that is touching the part. It returns the corresponding player instance or nil if it cannot find the player from the character. From here, we can access the player's PlayerGui.

--Get the player service.
local PS = game:GetService("Players")

script.Parent.Touched:Connect(function(hit)
    --Get the player from the character.
    local Player = PS:GetPlayerFromCharacter(hit.Parent)
    --If the player exists then change the visibility of the player's gui.
    if Player then
        Player.PlayerGui.ScreenGui.Frame.Visible = false
        Player.PlayerGui.Successpartone.Frame.Visible = true
    end
end)

Make sure it is a Script and not a LocalScript. Parent it under the brick.

Please accept the answer if this helps.

u have to get the gui trough the player

local plr = game:GetService("Players").LocalPlayer
game.workspace.part.Touched:Connect(function(hit) -- dont usee :connect its deprecated
        plr.PlayerGui.ScreenGui.Frame.Visible = false
        plr.PlayerGui.Successpartone.Frame.Visible = true
    end)