What went wrong here?

The problem is that 'countPlayers' will always be the same because you're not defining it inside the loop. So if, initially, 'countPlayers' wasn't less than or equal to one then it will never change.

Also, you cannot change a property from a variable directly. If you set a variable to a property, the variable becomes 'read only'. Set 'waitText' to the TextLabel, not the Text property of it.

And I have no idea what you were doing with the GetPlayerFromCharacter method.

local num = 1
waitText = workspace.mainGui.lobbyWait.TextLabel

while wait() do
    local countPlayers = game.Players.NumPlayers
    if countPlayers <= num then
        waitText.Text = "testing, testing, 123"
    end
end

3 Things Unintentionally Done With Your Script:

• Careful with while loops. You will need a "wait()," else Roblox will crash.

• countPlayers refer to the value of the NumPlayers property, not the property itself.

  • countPlayers will always be the number that it is assigned to, so if the server starts, the initial amount of players in the game is 0.

while game.Players.NumPlayers <= 1 do
    -- Code here
    wait()
end

• Same case for waitText.

while game.Players.NumPlayers <= 1 do
    waitGui.Textlabel.Text = "testing, testing, 123"
    wait()
end

What's the purpose of the first line? Delete that line out if you're not going to use it. This is for efficiency.