How do i make a player faster when a key is pressed?

The error is that you are using “250” instead of 250, the walk speed cannot be a string, but not only that, you are not specifying the key when they press, so any key will make the player sprint, but you need to specify an input function for user input service otherwise the script will never know the key they clicked

local playr = game.Players.LocalPlayer
local mouse = playr:GetMouse()
local uis = game:GetService("UserInputService")
local character = playr.CharacterAdded:Wait() -- Remove :wait(0.1)
character:WaitForChild("Humanoid")
character.Humanoid.WalkSpeed = 50 -- Use Numbers
local animationTrack = playr.Character.Humanoid:LoadAnimation(animation)

function run(key,gp)
    if key.KeyCode == Enum.KeyCode.Q and not gp then 
        script.Fwoosh:Play()
        game.Players.LocalPlayer.Character.Humanoid.WalkSpeed = 250
    end
end

function stop()
    game.Players.LocalPlayer.Character.Humanoid.WalkSpeed = 50
end

uis.InputBegan:connect(
    function(input)
        if input.KeyCode == Enum.KeyCode.Q then
            run(Q) -- you have to tell the script the key the player clicked
        end
    end
    uis.InputEnded:connect(
        stop() --no point of the key parameter since they will let go of the key
    )
)

Answer

Well, first, you can put in StarterPlayer considering it always updates your position even when the position is modified by a LocalScript. So something like this may work:

local uis = game:GetService("UserInputService")
local character = workspace:WaitForChild(script.Parent.Parent.Name)
local sprinting = false

--Camera options
local EasingSpeed = 3
local sprintFOV = 120
local defaultFOV = 70

--Speed options
local defaultRunSpeed = 16
local sprintRunSpeed = 32

--Jump options (optional)
local highJump = true
local defaultJumpPower = 50
local sprintJumpPower = 75

uis.InputBegan:Connect(function(input)
    if input.KeyCode == Enum.KeyCode.LeftShift then
        sprinting = true
    end
end)

uis.InputEnded:Connect(function(input)
    if input.KeyCode == Enum.KeyCode.LeftShift then
        sprinting = false
    end
end)


while wait() do
    if sprinting == true then
        if workspace.CurrentCamera.FieldOfView < sprintFOV + 1 then
            workspace.CurrentCamera.FieldOfView = workspace.CurrentCamera.FieldOfView + EasingSpeed
        end
        character.Humanoid.WalkSpeed = sprintRunSpeed
        if highJump == true then
            character.Humanoid.JumpPower = sprintJumpPower
        end
    elseif sprinting == false then
        if workspace.CurrentCamera.FieldOfView > defaultFOV - 1 then
            workspace.CurrentCamera.FieldOfView = workspace.CurrentCamera.FieldOfView - EasingSpeed
        end
        character.Humanoid.WalkSpeed = defaultRunSpeed
        character.Humanoid.JumpPower = defaultJumpPower
    end
end

If you need some things explained, continue to the next part.

Explanation So, if you look at the script, you can see many variables at the top. What these really mean is simple: EasingSpeed = The amount of FOV (Field of View) it changes every tick when sprinting sprintFOV = The FOV that is eased to when sprinting defaultFOV = The default FOV (FOV when not sprinting) defaultRunSpeed = The Humanoids WalkSpeed when not sprinting sprintingRunSpeed = The Humanoids WalkSpeed when sprinting highJump = whether or not the Humanoids JumpPower is affected by sprinting defaultJumpPower = the Humanoids JumpPower when not sprinting sprintingJumpPower = the Humanoids JumpPower when sprinting Feel free to customize these how you wish!

flunkywhat, Game Dev