My NumPlayer checking isn't working?

I'm guessing that well, LocalScripts run before the player is actually created. So you need to add this near the start of your script at about line 4:

repeat wait() until player and messageline and messagegui

Also, instead of doing WaitForChild for messagegui, do FindFirstChild. Because what I think is, your trying to find MessageLine in something that doesn't exist, like saying, "Get my phone in my backpack" when you don't have a backpack with you.

local player = game.Players.LocalPlayer
local messagegui = player.PlayerGui:FindFirstChild("Messages")
local messageline = messagegui:FindFirstChild("MessageLine")

repeat wait() until player and messageline and messagegui

while wait() do
    if game.Players.NumPlayers < 2 then
        messageline.Text = "Game about to start."
    else
        messageline.Text = "Waiting for 2 or more players."
    end
end

Since you have an infinite loop, you might want a break or use ChildAdded for the players service:

local player = game.Players.LocalPlayer
local messagegui = player.PlayerGui:FindFirstChild("Messages")
local messageline = messagegui:FindFirstChild("MessageLine")

repeat wait() until player and messageline and messagegui

while wait() do
    if game.Players.NumPlayers < 2 then
        messageline.Text = "Game about to start."
    else
        messageline.Text = "Waiting for 2 or more players."
    break
    end
end

or:

local player = game.Players.LocalPlayer
local messagegui = player.PlayerGui:FindFirstChild("Messages")
local messageline = messagegui:FindFirstChild("MessageLine")

repeat wait() until player and messageline and messagegui

game:GetService("Players").ChildAdded:connect(function(obj)
    if game.Players.NumPlayers < 2 and obj:IsA("Player") then
        messageline.Text = "Game about to start."
    else
        messageline.Text = "Waiting for 2 or more players."
    end
end)

If you want to start the game when there is 2 OR MORE players, instead of doing, NumPlayers < 2 switch the sign around, since how you set it up it says, "If there is less than 2 players" and add a = to the end so it says "If there is 2 or more players then..."

local player = game.Players.LocalPlayer
local messagegui = player.PlayerGui:FindFirstChild("Messages")
local messageline = messagegui:FindFirstChild("MessageLine")

repeat wait() until player and messageline and messagegui

game:GetService("Players").ChildAdded:connect(function(obj)
    if game.Players.NumPlayers >= 2 and obj:IsA("Player") then
        messageline.Text = "Game about to start."
    else
        messageline.Text = "Waiting for 2 or more players."
    end
end)

Hope this helps!(I don't know your error since at the time I am typing this you haven't told me the error yet, so tell me if there is still an error and I'll fix it!)