The outline won't be visible if you disable the UI (which is the button in the bottom right, you can also use F10). Make sure to capture T.I.T.S. either through game capture with transparency on or use Spout2
As a non-vtuber I also recommend using the scenes-menu to align the outline with your facecam location and keeping TITS in fullscreen in OBS, this way it looks like items fly through the screen towards you!