In the output it keeps saying 'SoundId cannot be assigned to'?

In the output it keeps saying 'SoundId cannot be assigned to'. I'm pretty sure I did the rest of the script correctly, why does it keep saying that? Here's the script:

local character = script.Parent

delay(0, function() 
	while wait() do
		if character.Humanoid.FloorMaterial  == Enum.Material.Sand then
			character.Humanoid.Running.SoundId = "rbxassetid://138080013"
			character.Humanoid.Running.Volume = 1	
		end	
	end
end)