Be able to move only when you jump?

local play = game.Players.LocalPlayer
local char = play.Character
local hum = char:WaitForChild("Humanoid")

if hum.Jump == true then hum.WalkSpeed = 16
elseif
hum.Jump == false then hum.WalkSpeed = 0

end

I have tried this script:

It doesn't seem to be working, does anyone know what I'm doing wrong?

###It did work. It only ran once though. You should put the code in an event. Events run each time something is triggered. In this case we will use the Humanoid.Jumping event.

local player = game:GetService("Players").LocalPlayer
local character = player.Character or player.CharacterAdded:Wait() -- wait for the character! 
local humanoid = character.Humanoid

humanoid.Jumping:Connect(function(active) -- active is if it's still jumping 
    while active do -- while it's jumping...
        humanoid.WalkSpeed = 16
        wait() -- without this, your script will hang! 
    end
    
    humanoid.WalkSpeed = 0 -- after the while loop breaks, code will run 
end)

###On a side note, always put your code in a code block. You do this by clicking the blue Lua icon and pasting the code in between the tildes. More info on that https://forum.scriptinghelpers.org/topic/82/how-to-format-questions-answers-on-the-main-site.