The Captain Sim 777 is a bit weird when it comes to the OPT/MAX altitudes. By that, I mean it rarely shows anything other than FL301/FL344. So this might explain why the CDU/FMC is giving you the UNABLE CRZ ALT message.

I fly at between FL310 and FL350. The one thing I do to help figure out how much fuel I need for the flight, is to complete the setup of the CDU, then check the PROG page to see how much fuel is expected to be remaining at my destination. I then use the Fuel and Payload > Fuel menu to adjust the fuel so that I will have somewhere in the vicinity of 20,000 - 25,000 pounds of fuel remaining (in the CDU, my reserves will be set at about 10.0 (10,000 pounds)). The main reason I will add an extra 10-15 thousand pounds is because
(1) I have to taxi to the runway on departure
(2) the CDU/FMC is not very accurate during the CLIMB and DESCENT
(3) a tailwind or headwind will increase or decrease, respectively, the remaining fuel upon arrival
(4) I have to taxi to the Gate

The system works really well for me and I use it in all my addon aircraft that have an CDU/FMC that will give me an estimate of the fuel that will be remaining at the end of the flight.