X cannot be assigned to?? (Help for plugin)

I want to create a plugin that'll help you position parts and do other stuff more easier than the way studio has What I want this script to do is to let you set the value and then change the position of the part to what you set

local player = game.Players.LocalPlayer
local mouse = player:GetMouse()
local playergui = player.PlayerGui
local screen = game.ReplicatedStorage.ScreenGui
local x = screen.Xie
local y = screen.Yie
local z = screen.Zie 
screen:Clone().Parent = playergui
placepart = function(key)
	local part = Instance.new("Part", workspace)
	part.Name = "CreatedPart"
	local setPosition = Instance.new("Vector3Value", part)
	setPosition.Name = "PartPosition"
	-- Set Position Values --	
	setPosition.Value.X = x.Text  --Here is the problem error "X cannot be assigned to"
	setPosition.Value.Y = y.Text  --Here is the problem
	setPosition.Value.Z = z.Text  -- Here is the problem
	part.Position = Vector3.new(setPosition.Value.X,setPosition.Value.Y,setPosition.Value.Z)		
		if key == "x" or key == "X" then
		game.ReplicatedStorage.ScreenGui:Clone().Parent = playergui
	end
end
mouse.KeyDown:Connect(placepart)

Hopefully you can answer my question :)

The positional properties of a Vector3 are read-only.

As you can see here, the coordinate properties of the Vector3 class are marked as read-only. This means that you can't set the X, Y and Z coordinates manually - you can only view/read each of their values. The correct way of creating a Vector3 is by using the Vector3.new() constructor and passing each coordinate value into it.

For example, as you are setting the value of a Vector3Value in your script, you would do the following:

setPosition.Value = Vector3.new(x.Text, y.Text, z.Text) -- Strings will automatically convert to numbers when necessary, so there's no need to use tonumber() here.

Your full script would look something like this (with a few minor tweaks):

local player = game.Players.LocalPlayer
local mouse = player:GetMouse()
local playergui = player:WaitForChild("PlayerGui") -- Waits for the PlayerGui to become available.
local screen = game.ReplicatedStorage:WaitForChild("ScreenGui") -- Wait for the ScreenGui to become available.
local x = screen.Xie
local y = screen.Yie
local z = screen.Zie 
screen:Clone().Parent = playergui
local function placePart(key) -- Small rearrangement of the function's creation so that it's easier to read.
    local part = Instance.new("Part", workspace)
    part.Name = "CreatedPart"
    local setPosition = Instance.new("Vector3Value", part)
    setPosition.Name = "PartPosition"
    -- Set Position Value --   
    setPosition.Value = Vector3.new(x.Text, y.Text, z.Text)
    part.Position = setPosition.Value -- The value is already a Vector3, you don't need to create a new one to set the part's position.        
        if key == "x" or key == "X" then
        game.ReplicatedStorage.ScreenGui:Clone().Parent = playergui
    end
end
mouse.KeyDown:Connect(placepart) -- It is better to use UserInputService for new work as this is deprecated.

I would also consider using the UserInputService to handle your key bindings as opposed to KeyDown as it is deprecated and should not be used for new work.

I hope my answer helped you. If it did, be sure to accept it.