local StarterGui = game:GetService("StarterGui")
local players = game:GetService('Players')

local function teamChanged(player)
	player:GetPropertyChangedSignal('Team'):Connect(function()
		wait()
		local Success = pcall(function()
			StarterGui:SetCore("ChatMakeSystemMessage", 
				{
					Text = player.Name .. ' has progressed to ' .. player.Team.Name,'All'; --The chat message
					Color = player.Team.TeamColor.Color; --Chat message color, defaults to white
					Font = Enum.Font.SourceSansBold; --Chat message font, defaults to SourceSansBold
					TextSize = 18 --Text size, defaults to 18
	
			})
		end)
	end)
end

players.PlayerAdded:Connect(teamChanged)

for _, player in pairs(players:GetPlayers()) do
	teamChanged(player)
end

I have placed it in StarterGui as localScript and no errors occur.

Anonymous