By trying adding it it apperas on the components list and nothing is showing on the 3D view.
Also, it's shown as follows on the 2D view :
In current AutoPIPE Vessel Version (43.01.01.11) we are unable add a platform above a horizontal vessel. At this time platforms can only be added to Vertical vessels. A new feature (638819) has been logged and should appear in a future version of the program.
Senior Technical Support EngineerBentley Systems Inc