Why won't the TextBox change it's Text?

It's actually a quick fix, I've done the same issue myself in one of my own loops. (This assumes the script is in the workspace)

local StarterGui = game:GetService("StarterGui")
local TextBox = StarterGui.ScreenGui.TextBox

game.Players.PlayerAdded:Connect(function(player)

for i = 1,60,1 do
    TextBox.Text = i
    wait(0.1) break
end
end)

You have a "break" after the "wait(0.1)" which ends the loop right away after the first iteration, just remove it like this:

local StarterGui = game:GetService("StarterGui")
local TextBox = StarterGui.ScreenGui.TextBox

game.Players.PlayerAdded:Connect(function(player)

for i = 1,60,1 do
    TextBox.Text = i
    wait(0.1)
end
end)

That should do the fix, I didn't fully check the script for any other issues. (Before my relook)

Now after a relook, there's also another issue that other users were talking about in the comments, it doesn't update for other players. I also have a QUICK fix for that!

local Players = game:GetService("Players")
local StarterGui = game:GetService("StarterGui")
local TextBox = StarterGui.ScreenGui.TextBox

game.Players.PlayerAdded:Connect(function(player)

for i = 1,60,1 do
    for i,plr in pairs(Players:GetPlayers()) do
        local TextBox = plr.PlayerGui.ScreenGui.TextBox
        if TextBox then
            TextBox.Text = i
        end
    end
    wait(0.1)
end
end)

And to add to it, I think you're trying to get the amount of players, that is really easy to get!

game.Players.PlayerAdded:Connect(function(player)
    playercount = #game.Players:GetPlayers()
    for i,plr in pairs(game.Players:GetPlayers()) do
        local TextBox = plr.PlayerGui.ScreenGui.TextBox
        if TextBox then
            TextBox.Text = i
        end
        wait(0.1)
    end
end)

Sorry my answer is all over the place, but that last one should work!

You can't use a playeradded event inside of a gui or edit the startergui. Use a localscript inside of the textlabel with the following text

local TextBox = script.Parent
local iscounting = true
while iscounting do
    wait(1)
    if TextBox.Text ~= 0 then
        TextBox.Text = TextBox.Text - 1
    end
end