So, when I test it in studio, it works fine, but in-game it doesn't work. The server is a personal server if you wonder what type of place it is.
Player = game.Players.LocalPlayer
Character = Player.Character
Torso = Character.Torso
Mouse = Player:GetMouse()
hold = false
Mouse.Button1Down:connect(function()
if hold == false then
hold = true
while hold == true do
wait(0.00001)
Player.Character.Humanoid.WalkSpeed = Player.Character.Humanoid.WalkSpeed +1
wait(0.1)
local Brick = Instance.new("Part", Character) --Changed Player to Character Brick.Anchored = true
Brick.Position = Torso.Position + Vector3.new(5,0, 0)
Brick.BrickColor = BrickColor.new("New Yeller")
Brick.Size = Vector3.new(3,5,4)
Brick.Anchored = true
Brick.Transparency = 0.5
Brick.Reflectance = 0.4
game.Debris:AddItem(Brick,1)
Brick.TopSurface = 0
Brick.BottomSurface = 0
Brick.CanCollide = false
local smoke = Instance.new("Smoke", Brick)
smoke.Opacity = 0.3
smoke.RiseVelocity = 0.4
smoke.Size = 0.5
smoke.Color = Color3.new(226,226,36)
smoke.Enabled = true
local fire = Instance.new("Fire", Brick)
fire.Enabled = true
fire.Heat = 10
fire.Color = Color3.new(255,173,0)
fire.SecondaryColor = Color3.new(255,0,0)
fire.Size = 15
end
end
end)
Mouse.Button1Up:connect(function()
if hold == true then
hold = false
Player.Character.Humanoid.WalkSpeed = 16
end
end)

5 comments
Is this a local script?
yes
If FilteringEnabled is enabled this script will not work since it is a LocalScript trying to edit global objects.
Where are global objects?
Global objects being objects in the workspace or other server-restricted locations that every player can see such as characters in your case. FilteringEnabled can be toggled in the properties list of the Workspace.