Ok so what is ROB.

And i entered 15.9 as reserve.

I entered 281,000 of fuel.

ROB=Remaing On Board.

The value of ROB fuel is constantly calculated and updated by the FMC during your flight and is displayed on the PROG page in front of you destination airport.
During the flight you should monitor it and check that it remains consistent with your flight plan. If that value goes below the reserve value you entered in the Init page, then the message "Insufficient fuel" is triggered.

As you entered 15.9 in the reserve and your planned ROB is 36.9, it means that if you have the message "Insufficient fuel" displayed, the FMC predicts that your calculated ROB is below the 15.9 and therefore there is a significant discrepancy between the FMC calculation and the flight plan.

Did you check that the total distance of the trip calculated by the FMC matches the one in the flight plan? It may happen that a waypoint wrongly entered in the FMC (typo for instance) may be significantly off the route and therefore the distance calculated by the FMC is way higher than the one on the flight plan.

During the preflight, when you fill the FMC, you should double check the following:

_ ZFW and fuel quantity entered in the FS options of the CDU matching the flight plan

_ Once the route is entered (including at least departure), the calculated distance in the FMC matches the flight plan 

_ Once the Init page filled, the ZFW, fuel quantity, CI, and reserve values match the flight plan.

_ Once the winds aloft are uploaded, the calculated ROB matches also the flight plan and you have no Insufficient fuel message displayed (ROB above the reserve fuel entered).

If there is any significant discrepancy, you should correct them before departure. 

fuel remaining is probably your problem

 

you also need to check the weather map in pfpx and look for possible weather that could mean increased headwinds in the cruise phase, and to a lesser extent the descent phase.

 

if a crosswind shifts to a headwind at some point of the cruise it could significantly affect your fuel burn and result in you burning through your planned reserve fuel when reaching the destination.

 

your reserve should be enough fuel to reach the destination, go around, divert and go around once. i don't think you need 16tons of fuel to do that in most cases, but anyway, when you make your flightplan make sure you input a realistic reserve fuel figure. keep in mind pfpx default flightplan generation options automatically include basic reserve fuel so anything you add on top of that is just extra and is reducing your fuel economy if you add on top of it (you're carting around an extra 10+ tons of fuel when pfpx already planned in adequate reserve fuel).