How to use a keyDown event in another one?[Solved]

Well, one way you could approach this issue is by setting a Boolean to (true/false) every time the "b" key is pressed, and based on what the value is, code whatever you want to happen.

Tips:

Since you can only get the player's mouse via LocalScripts, you can just get the player using game.Players.LocalPlayer

The player's character might take a little longer to completely load, therefore, an easier way to wait for the character is by using Player.CharacterAdded:Wait()

local player = game:GetService('Players').LocalPlayer
local character = player.Character or player.CharacterAdded:wait()

local mouse = player:GetMouse()
local bool = false

mouse.KeyDown:connect(function(key)
    if key:lower()== 'b' then
        bool = not bool

        if bool and wait() then
            print(true)
        else
            print(false)
        end
    end
end)

I re-named a few variables for and spaced it all out a bit just to keep it all clean and easier to understand.

You don't need to do that. Really, all you have to do is use variables to keep track of whether or not the part is there. Just a suggestion, it should be easy to implement, if you need me to show you how to do that just comment.

Player = script.Parent.Parent
c = Player.Character
mouse = Player:GetMouse()
activate = false
local partIsActivated = false
function onMouseButton1Down(Key)
    activate = true
    key = Key:lower()
    if key == "b" then
    if not partIsActivated then
    c["Right Arm"].Transparency = 1
    x = Instance.new("Part")
    x.TopSurface = "Smooth"
    x.BottomSurface = "Smooth"
    x.Size = c["Right Arm"].Size
    x.BrickColor = BrickColor.new("Black")
    x.Transparency = 0
    x.Reflectance = 0.1
    x.Parent = c
    w1 = Instance.new("Weld", c)
    w1.Part0 = w1.Parent["Right Arm"]
    w1.Part1 = x
    w1.C1 = CFrame.fromEulerAnglesXYZ(0, 0, 0) *CFrame.new(0, 0, 0)\
    elseif partIsActivated then
    --put the code here to remove the part, or you can make it so the part is just visible or invisible. up to you
    end
    end 
end
mouse.KeyDown:connect(onMouseButton1Down)

I've fixed the script that should make the toggle work. It should allow you to toggle the arm and the real roblox arm.

Player = script.Parent.Parent
c = Player.Character
mouse = Player:GetMouse()
activate = false

function onMouseButton1Down(Key)
    local key = Key:lower()
    if key == "b" then
    if activate == false then
    activate = true
    c["Right Arm"].Transparency = 1
    local x = Instance.new("Part")
    x.TopSurface = "Smooth"
    x.BottomSurface = "Smooth"
    x.Size = c["Right Arm"].Size
    x.BrickColor = BrickColor.new("Black")
    x.Transparency = 0
    x.Reflectance = 0.1
    x.Parent = c
    x.Name = "FakeArm"
   local  w1 = Instance.new("Weld", c)
    w1.Part0 = w1.Parent["Right Arm"]
    w1.Part1 = x
    w1.C1 = CFrame.fromEulerAnglesXYZ(0, 0, 0) *CFrame.new(0, 0, 0)
    else
    activate = false
    c.FakeArm:remove()
     c["Right Arm"].Transparency = 0
    end
end
end
mouse.KeyDown:connect(onMouseButton1Down)