How does one set an arm on fire?

local Player = game.Players.LocalPlayer
local Mouse = Player:GetMouse()

Mouse.KeyDown:connect(function(Key)
	Key = Key:lower()
	if Key == "z" then
		print ("Test")
		local fire = Instance.new('Fire')
	end
end)

I'm new to scripting and was wondering how to put the fire onto the player's left arm. What I am trying to do is make it so if the player has pressed the "z" key, their left arm will set on fire. The script above is what I have so far. Whenever I try and do it, the script breaks and in the output it says "Player is not a valid member of humanoid".

I don't think that the error you are getting is part of the script above as you do not access the humanoid in the characters model.

We now have a whole service to manage the users input called UserInputService which is why KeyDown and KeyUp deprecated so please do not use them.

As you have posted a script I will give an example, this also uses a remote event as you should have FilteringEnabled:-

Server script:-

-- add the event (You do not need to create this in a script, you can manually add it)
local remoteFire = Instance.new('RemoteEvent', game:GetService('ReplicatedStorage')) -- create a remote event object and adds it to the rep storage so both the server and client can see it
remoteFire.Name = 'remoteFire'

remoteFire.OnServerEvent:connect(function(plr, fire) -- the 1st arg is passed automatically followed by all of the other args send by the client
	local char = plr.Character  -- the character is the model in the workspace this could be nil
	
	if char then -- check for a nil character
		
		-- we could also check that the player does not have the fire else we could be adding more? you can look into this		
		
		-- fire is a boolean passed to determine if we add or delete the fire
		if fire then
			-- add fire
			if char['Left Arm'] then
				local fire = Instance.new('Fire', char['Left Arm'])
				-- set other things for the fire ect
				print('Fire added')
			else
				print('Character model does not have a left arm')
			end
		else
			-- remove fire, this should be simple to add
		end
	else
		print('Character model is nil')
	end
end)

This is a local script in StarterPlayerScripts:-

local usrInpServ = game:GetService('UserInputService') -- this can only be used in a local script the server does not have any access to the clients inputs
local remoteFire = game:GetService('ReplicatedStorage'):WaitForChild('remoteFire') -- the name of the event we created on the server side
local deb = true

usrInpServ.InputBegan:connect(function(inputObj, gameProcEvent)
	if deb then -- this will help with stopping events are we do not want to spam the remote 
		-- it is good practice to use "Enums" instead of values or ints
		if inputObj.UserInputType == Enum.UserInputType.Keyboard then -- only run with keyboard
			if inputObj.KeyCode == Enum.KeyCode.Z then
				deb = false -- we now can process so stop other requests
				remoteFire:FireServer(true) -- we do not pass the player as the server knows what player fired the event
				
				-- i dont know how you want to handle multiple requests
				-- atm this will only run once
				-- we could add a cool down
				wait(10)
				deb = true
			end
		end
	end
end)

I hope this helps, please comment if you do not understand how/why this script works.

local rightArm = Player.Character["Right Arm"]
local leftArm = Player.Character["Left Arm"]

local fire = Instance.new("Fire")
fire.Parent = rightArm
fire.Parent = leftArm

This will put the fire in the arm :)