void GUI(BasePlayer player, string msg)
        {

            CuiHelper.DestroyUi(player, "PlayerID");

            var elements = new CuiElementContainer();
            var GUI = elements.Add(new CuiPanel
            {
                Image =
                {
                    Color = "0 0 0 0.3"
                },
                RectTransform =
                {
                    AnchorMin = "0.66, 0.957",
                    AnchorMax = "0.802, 1"
                },
                CursorEnabled = false
            }, "Overlay", "PlayerID");

            elements.Add(new CuiElement
            {
                Parent = "PlayerID",
                Components =
                    {
                        new CuiTextComponent
                        {
                            Text = $"{player.userID}",
                            Color = "",
                            FontSize = 14,
                            Align = TextAnchor.MiddleCenter
                        },
                        new CuiRectTransformComponent
                        {
                            AnchorMin = "0 0",
                            AnchorMax = "1 1"
                        }
                    }
            });

            elements.Add(new CuiLabel
            {
                Text =
                    {
                        Text = "Test",
                        FontSize = 12,
                        Align = TextAnchor.MiddleCenter
                    },
                RectTransform =
                    {
                        AnchorMin = "0.175 0",
                        AnchorMax = "1 1"
                    }
            }, GUI);

            CuiHelper.AddUi(player, elements);
        }
        void OnPlayerInit(BasePlayer player)
        {
            GUI(player, player.UserIDString);
        }

        [ChatCommand("t")]
        void TestCommands(BasePlayer player, string cmd, string[] args)
        {
            GUI(player, player.UserIDString);
        }

Kick Reason: RPC Error in AddUI: Input string was not in a correct format.

I suppose text color should be set but it's not. Just don't set it and leave the default value :)