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)