I'm BlackOrange and I will be helping you today!

Issues:

Your biggest issue is misunderstanding why we use RemoteEvents and also what we should do on the Client and on the Server.

1. This issue is really big. You are constantly firing a remote to the server in a LOOP this basically spells death on your game

2. You got some stuff in there that you shouldn't and there are better ways to do things.

Solution:

Alright, let's think. Do you really want the Server to put a selection box around a part? When the server does it, it's not only the player that is seeing it. The ENTIRE server including all the players can see it. This could be chaotic. What if all the players hovered over something, all of them would be selected from the Client. I don't see why you would do this in the first place.

Alright, so for this "Project" / "Task", you will NOT be needing a ServerScript. All you need is a LocalScript inside of StarterCharacterScripts in StarterPlayer

Let's take a look at wiki: https://www.robloxdev.com/api-reference/class/SelectionBox

So how would you be able to fix your problem?

Here are the steps:

1. Remove the remote event

2. Re write everything but this time do it on the client only

How would this be beneficial?

Well

1. You aren't killing your game

2. You aren't using a loop

3. It would look much smoother on the client

So yea, do it on the client. Remember when you gotta do stuff and why.

Best of luck developer! hoped this helped!

Part 1: How I would do it (skip this if you want the answer to your question) Hey, so instead of creating a new part for the selection box, let's use an instance called SelectionBox.

Let's start by creating this instance and placing it inside StarterGui.

Now that you've created the instance you might be wondering how I can connect it to a part, to do that you just have to set it's Adnornee to the part.

Using your client script we can do this.

play = game.Players.LocalPlayer
mouse = play:GetMouse()
local selectionbox = play.PlayerGui:WaitForChild("SelectionBox") --also gatta wait for things to load :p

if mouse.Target ~= nil then
    while wait() do
    if target.Name ~= "Baseplate" then
        selectionbox.Adornee=mouse.Target
    end
        wait()
    end
end

Part 2: Answer to your question

For the LocalScript we need to remove the check for mouse.Target BEFORE the actual loop starts. Once the loop starts we can check to see if mouse.Target is nil and the target's name is not "TargetPart".

play = game.Players.LocalPlayer
mouse = play:GetMouse()

while wait() do
    if mouse.Target and mouse.Target.Name~="TargetPart" then
        game.ReplicatedStorage.Target:FireServer(mouse.Target)
    end
    wait()
end

Let's destroy the -

SERVERSCRIPT For destroying Part, Parented to the Part. We only need to deal with the main server script now.

In the ServerScript we begin by setting curPart as a global variable.

.Next we can set curPart to be a clone of TargetPart and set that up and if target is nill or doesnt pass our first if statement, we can Destroy() curPart if it exists to remove it once you move your mouse off the part.

local curPart
game.ReplicatedStorage.Target.OnServerEvent:Connect(function(player, target)
    if target == nil then return end
    if not target:FindFirstChild("TargetPart") and target.Name ~= "TargetPart" and target.Name ~= "Baseplate" then
        curPart = game.ReplicatedStorage.TargetPart:Clone()
        curPart.Size = target.Size + Vector3.new(0.2,0.2,0.2)
        curPart.Anchored = true
        curPart.CFrame = target.CFrame
        curPart.Parent = target
        print(target.Name)
    elseif curPart then
        curPart:Destroy()
    end
end)

Please ask questions if you have any.