Running speed not working?

When a player buys a gamepass, they don't have a walkspeed of 21.

local passId = 278389550 -- change this to your game pass ID.

function isAuthenticated(player)
    return game:GetService("GamePassService"):PlayerHasPass(player, passId)
end

game.Players.PlayerAdded:connect(function(plr)
    if isAuthenticated(plr) then
        plr.Character.Humanoid.WalkSpeed = 21
    end
end)