How to make an animation play and not stop when a player is walking?

Basically you make an animation firstly using animation editor, next you export it and get the id and then use:

local Players = game:GetService("Players")

local function onCharacterAdded(character)
    local humanoid = character:WaitForChild("Humanoid")

    for _, playingTracks in pairs(humanoid:GetPlayingAnimationTracks()) do
        playingTracks:Stop(0)
    end

    local animateScript = character:WaitForChild("Animate")
    animateScript.run.RunAnim.AnimationId = "rbxassetid://616163682"        -- Run
    animateScript.walk.WalkAnim.AnimationId = "rbxassetid://616168032"      -- Walk
    animateScript.jump.JumpAnim.AnimationId = "rbxassetid://616161997"      -- Jump
    animateScript.idle.Animation1.AnimationId = "rbxassetid://616158929"    -- Idle (Variation 1)
    animateScript.idle.Animation2.AnimationId = "rbxassetid://616160636"    -- Idle (Variation 2)
    animateScript.fall.FallAnim.AnimationId = "rbxassetid://616157476"      -- Fall
    animateScript.swim.Swim.AnimationId = "rbxassetid://616165109"          -- Swim (Active)
    animateScript.swimidle.SwimIdle.AnimationId = "rbxassetid://616166655"  -- Swim (Idle)
    animateScript.climb.ClimbAnim.AnimationId = "rbxassetid://616156119"    -- Climb
end

local function onPlayerAdded(player)
    player.CharacterAppearanceLoaded:Connect(onCharacterAdded)
end

Players.PlayerAdded:Connect(onPlayerAdded)

credit to the dev form: https://developer.roblox.com/en-us/articles/using-animations-in-games

If you want the animation to play when the character stops walking, simply set the priority of the animation to idle and play the animation however you decide and whenever the character isn't moving and doesn't have a higher priority animation using the parts used for that animation it'll play the animation (if that make sense)