Making all players frozen?

So I am making a game with rounds and I and trying to make it where the players can't get ahead before the round actually starts. So I am trying to make it to where they can't move until the countdown has finished.

This is pretty much all I have so far but it isn't working and I'm sure it's far from correct and I'm not sure where to look.

for _, player in pairs(game.Players:GetPlayers()) do
    if player.Character then
        player.Character.Humanoid.Walkspeed = 0
    end
end