Have you checked that your piping family has a diameter 15 available? if not you need to add it in manage>mechanical settings.
Another thing is the connectors are vertical so revit wants a vertical pipe on them, instead of clicking on the connector and start drawing (horizontal) try to right-click one, choose draw from and change the elevation in the interactive menu and select apply. (don't click in the drawing!)
Another easy way to connect your sink is to first draw your main pipe horizontally below the sink or in the wall, then select the sink > click the "connect into" button and select the nearest pipe, revit will try to connect automatically like this. Disadvantage of this method is you don't have complete control on how the pipes are drawn.
Good luck!