Run animation plays while standing still fix?

How would I make it so you could only run when you are moving?

wait(1)
local UIS = game:GetService("UserInputService")
local p = game:GetService("Players").LocalPlayer
local c  = p.Character or p.CharacterAdded:Wait()
local anim = Instance.new("Animation")
anim.AnimationId = "rbxassetid://4588222471"
local RunAnimation = c:WaitForChild("Humanoid"):LoadAnimation(anim)
UIS.InputBegan:Connect(function(i,a)
    if not a then
        if i.KeyCode == Enum.KeyCode.LeftShift then
            c:WaitForChild("Humanoid").WalkSpeed = 20
            RunAnimation:Play()
        end
    end
end)

UIS.InputEnded:Connect(function(i,a)
    if not a then
        if i.KeyCode == Enum.KeyCode.LeftShift then
            c:WaitForChild("Humanoid").WalkSpeed = 13
            RunAnimation:Stop()
        end
    end
end)

There's nothing wrong with your code. Try setting the animation's priority to "Action".

You should check if the player is moving, and add that into your if statement!

local velocity = math.ceil(c.HumanoidRootPart.Velocity.magnitude)

Check if velocity > 3 (means they're actively moving), and then allow the sprint.

wait(1)
local UIS = game:GetService("UserInputService")
local p = game:GetService("Players").LocalPlayer
local c  = p.Character or p.CharacterAdded:Wait()
local anim = Instance.new("Animation")
anim.AnimationId = "rbxassetid://4588222471"
local RunAnimation = c:WaitForChild("Humanoid"):LoadAnimation(anim)
UIS.InputBegan:Connect(function(i,a)
    if not a then
        local velocity = math.ceil(c.HumanoidRootPart.Velocity.magnitude)
        if i.KeyCode == Enum.KeyCode.LeftShift and (velocity > 3) then
            c:WaitForChild("Humanoid").WalkSpeed = 20
            RunAnimation:Play()
        end
    end
end)

UIS.InputEnded:Connect(function(i,a)
    if not a then
        if i.KeyCode == Enum.KeyCode.LeftShift then
            c:WaitForChild("Humanoid").WalkSpeed = 13
            RunAnimation:Stop()
        end
    end
end)