What does "Attempt to index nil with 'Character' " mean?

to get all players use for loop to get all players

while true do
    local status = game.ReplicatedStorage.Status
    local inter = game.ReplicatedStorage.Intermission
    status.Value = "Time until next round: "
    inter.Value = 30
    repeat
        inter.Value -= 1
        wait(1)
    until inter.Value == 0
    local Josh = game.ReplicatedStorage.Spawn:Clone()
    Josh.Parent = workspace
for i, plr in pairs(game.Players:GetPlayers()) do
    local Character = plr.Character
        if Character then
            Character.HumanoidRootPart.CFrame = Josh.CFrame
            plr.Humanoid.WalkSpeed = 0
        end
end
    status.Value = ""
    inter.Value = 3
    wait(1)
    inter.Value = 2
    wait(1)
    inter.Value = 1
    wait(1)
    inter.Value = 0
    status.Value = "Go!"
    wait(1)
    status.Value = ""
    Josh:Destroy()
    for i, plr in pairs(game.Players:GetPlayers()) do
    local Character = plr.Character
        if Character then
            plr.Humanoid.WalkSpeed = 16
        end
end
end

You can use LocalPlayer unless you're in a local script. If this is a local script, change it right now because you are screwing up the code for other players by having multiple local scripts. If you want to do what you are doing, use a for loop that loops through the players characters that teleports them.

You are unable to recieve the "LocalPlayer" by doing game.Players.LocalPlayer on a ServerScript, as that runs on the server.

There are multiple ways of receiving the player, for example, you could use events with parameters that returns the player.

EXAMPLE

game.Players.PlayerAdded:Connect(function(plr)
    player = plr
end)