The plane needs sufficient runway space to get all of that fuel in the air for that ULH flight.
Pay attention to the big red box on the right when you try to set up the route.
RTB's longest runway is 2 240 x 45 m (Asphalt)
772 ER Runway requirement (MTOW) 2 820 m (Basic weight variant)
So, the plane has to take off 'light' with either fewer pax or fuel. In this case probably neither.