How to make Player Invisible Then Visible When pressing a key 2 times?

First of all, Make a local script and put it in StarterPack or StarterPlayerScripts

local UIS = game:GetService("UserInputService") -- Get user input service
local Visible = true
local player = game.Players.LocalPlayer
UIS.InputBegan:Connect(function(key) -- when player presses a key
if key.KeyCode == Enum.KeyCode.R then -- Change R to the key you want
    if Visible == true then
game.ReplicatedStorage.RemoteEvent1:FireServer() -- We have to fire server
Visible = false
    elseif Visible == false then
        game.ReplicatedStorage.RemoteEvent2:FireServer()
        Visible = true
    end
end
end)

You need to make 2 RemoteEvents. Put them both in ReplicatedStorage. Name the first "RemoteEvent1" And the second "RemoteEvent2"

Now create a server script ("script") and put it in ServerScriptService

game.ReplicatedStorage.RemoteEvent1.OnServerEvent:Connect(function(plr)
    for i,v in pairs(game.Workspace[plr.Name]:GetDescendants()) do
        if v:IsA("Part") or v:IsA("MeshPart") then
            v.Transparency = 1 -- this makes them invisible
        end
    end 
end)

Then make another server script and put it in the same place. Make the code for that script

game.ReplicatedStorage.RemoteEvent2.OnServerEvent:Connect(function(plr)
    for i,v in pairs(game.Workspace[plr.Name]:GetDescendants()) do
        if v:IsA("Part") or v:IsA("MeshPart") then
            v.Transparency = 0
        end
    end 
end)

I've tested this out and it worked, enjoy

you have to use userinputService. You also need a remote event if you want everyone to see your character invisible,

localScript in startercharacterScripts

local player = game.Players.LocalPlayer
local character = script.Parent -- Since the script will be inside the character, easier way is to use script.Parent
repeat wait() until character:FindFirstChild("Humanoid")
local UserInputService = game:GetService("UserInputService")
local isInvisible = false

UserInputService.InputBegan:Connect(function(input,gameProcessedEvent) -- input is the button the player pressed and gameProcessedEvent makes sure that you are not chatting, since if you dont use it players will accidentally press a button while chatting
    if input.KeyCode == Enum.KeyCode.L  then-- Your button for making the character invisible
        game:GetService("ReplicatedStorage").RemoteEvent:FireServer(isInvisible)
    end
end)

Server Script:

local ReplicatedStorage = game:GetService("ReplicatedStorage")
local RemoteEvent = ReplicatedStorage:WaitForChild("RemoteEvent")

RemoteEvent.OnServerEvent:Connect(function(player,isInvisible) -- In Server Event, the first parameter is always the player
    if not isInvisible then
        isInvisible = true
        for _,v in pairs(player.Character:GetDescendants()) do -- Descendants are ALL the parts including the children of the children
            if v:IsA("BasePart") or v:IsA("Decal") then -- Because parts and decals both have transparency, this will make the script shorter
                v.Transparency = 1
            elseif v:IsA("Accessory") then
                v:FindFirstChild("Handle").Transparency = 1
            end
        end
    elseif isInvisible then
        isInvisible = false
        for _,v in pairs(player.Character:GetDescendants()) do
            if v:IsA("BasePart") or v:IsA("Decal") then
                v.Transparency = 0
            elseif v:IsA("Accessory") then
                v:FindFirstChild("Handle").Transparency = 0
            end
        end
    end
end)