I would do this in order to make this work, and then you can customize it as you like.
Create a new animation using the Animation Tool in Roblox Studio.
Set the animation in Loop and Action, IN the Animation Tool in Studio first.
Place the animation in ServerStorage maybe, and instance from there, I prefer using the animation ID in the script.
Then place this script inside the Rig:
wait(10) -- Wait 10 seconds before start animation
print("animation starting")
local humanoid = game.Workspace.NAME OF YOUR RIG.Humanoid -- Type the name of ur Rig
local anim = Instance.new("Animation")
anim.AnimationId = "rbxassetid://your animation id" -- Type the animation ID of ur animation
local animTrack = humanoid:LoadAnimation(anim)
animTrack.Looped = true
animTrack.Priority = Enum.AnimationPriority.Action
animTrack:Play()