How do i get the left arm of my character?

Hey! This seems like an issue because you are referring to a part that does not exist on R15 avatars. If you are using R6, just let me know and I can try to figure out the issue from there.

When Roblox changed to R15, it turned both arms into 3 separate parts to allow for more freedom with animation. These parts are LeftUpperArm, LeftLowerArm, and LeftHand.

To make the entire left arm neon, you can do this:

local players = game.Players.LocalPlayer
local character = players.Character
local UIS = game:GetService("UserInputService")

UIS.InputBegan:Connect(function(key)
    if key.KeyCode == Enum.KeyCode.T then
        character["LeftUpperArm"].Material = Enum.Material.Neon
        character["LeftLowerArm"].Material = Enum.Material.Neon
        character["LeftHand"].Material = Enum.Material.Neon
    end
end)

EDIT: As you are using R6, this issue is because there is a space in "Left Arm".

To refer to an object with a space, you should use character["Left Arm"] as opposed to character.Left Arm.

local players = game.Players.LocalPlayer
local character = players.Character
local UIS = game:GetService("UserInputService")

UIS.InputBegan:Connect(function(key)
    if key.KeyCode == Enum.KeyCode.T then
        character["Left Arm"].Material = Enum.Material.Neon
    end
end)

Thernus's Answer is pretty straight forward except for the fact that what if you wanted to use R6 and R15?

Thernus is not wrong in any sort of way I am just going to show you what you can incorporate for improvement a humanoid has a property called RigType you can actually detect if they are either R6 or R15 and you would do it like this

local player = game.Players.LocalPlayer
local character = player.Character or player.CharacterAdded:Wait()

-- Have this inside your inputbegan
local humanoid = character:FindFirstChild("Humanoid")
if humanoid then
    if humanoid.RigType == Enum.HumanoidRigType.R6 then -- Is a R6 character.
        -- Assign variables to the body parts in R6; do desired task
    elseif humanoid.RigType == Enum.HumanoidRigType.R15 then -- Is a R15 character.
        -- Assign variables to the body parts in R15; do desired task
    end
end

RigType

Enum reference

If this helped an upvote or solved would be greatly appreciated. Good luck!

• Best Regards, -Syn